Chris Concannon
President and Chief Operating Officer at MarketAxess
Thanks, Rick.
Slide 5 provides an update on Open Trading and product expansion. Open Trading continues to support credit market liquidity by offering all participants the chance to engage with the market. In the third quarter, over 25,000 orders and $13 billion in notional value was available daily through our Open Trading marketplace.
Dealers have also realized the benefits of Open Trading and are increasingly seeking anonymous all-to-all liquidity. Dealer RFQ volume grew 20% year-over-year to $59 billion during the quarter. The increased diversity of participation continues to drive cost saving opportunities. Despite compressed credit market spreads, clients saved an estimated $121 million in track transaction cost during the quarter due to price improvements delivered by Open Trading. The acquisitions of LiquidityEdge and MuniBrokers highlight our investment in new markets and the growing application of Open Trading across the fixed income landscape. $910 billion of US Treasury trading volume was executed on MarketAxess in the quarter, up 22% from the prior year.
We have made several enhancements to our rates trading offering in recent months, including our launch of all-to-all click-to-trade functionality and extending RFQ trading capabilities for client to dealer orders. The expansion of Open Trading for US Treasuries is a critical priority for us and aligns with recent G30 recommendations for an all-to-all marketplace in treasuries. Municipal bond trading on MarketAxess grew 92% to $5.4 billion in the quarter, an additional $17.4 billion in volume was conducted through MuniBrokers, our inter-dealer electronic platform, which we currently do not include in our muni bond volume totals.
Integration efforts of the MuniBrokers platform are well underway and we are targeting the fourth quarter for the initial phase of our integration of the platform into our Open Trading network. We believe these investments in government bond and municipal bond trading solutions will add approximately 25% to our long-term addressable market opportunity.
Slide 6 highlights the growing momentum of automation in credit trading. Automated trading on MarketAxess reached new records in the quarter, growing to $42 billion in volume and over 224,000 trades. 115 firms leveraged our automated trading protocols in the quarter, up from 86 last year. Today, Auto-X represents 19% of total trade count and 7% of our total volume. The use of dealer algorithms is continuing to grow on the platform with approximately 4.4 million algo responses in the third quarter, up 17% from the same period last year.
The growth of dealer algorithms and our automated trading tools are driving a steady increase in the average number of responses on MarketAxess. In the third quarter, we saw an average of 7 responses per inquiry versus 5.8 in the third quarter of 2020. This demonstrates enhancements in our liquidity as a result of the increased engagement from our diverse investor and dealer community.
Slide 7 demonstrates the growth from diversifying our business initiatives. The acquisition of Regulatory Reporting Hub helped drive total post-trade services revenue to $9.4 million in the quarter, up 101% year-over-year. The addition of Continental European clients to our suite of Regulatory Reporting services through this acquisition has further bolstered our unique data solutions. Through these types of post-trade data sources, we have seen sizable benefits to our data solutions like Axess All and Composite+. Both Axess All and Composite+ helped drive our information services revenue to $9.6 million in the quarter, which is up 13% year-over-year. Combined information services and post-trade revenue now account for 12% of total revenues, up from 8% in the third quarter of 2020.
Following enhancements to our portfolio trading solution in May, we have seen significant traction with our new functionality. 54 unique investor firms and 13 dealers were active since May and drove record volume of $8.9 billion in the quarter. We believe our active client group is the same group of participants active in the market wide portfolio trading today. Activity in our session based protocol Mid-X reached record volumes in the quarter of $3.4 billion. We plan on expanding Mid-X beyond Eurobonds to US credit later this year.
Now let me turn the call over to Chris to provide an update on our financials.