Richard M. McVey -- Chairman & Chief Executive Officer
Good morning, and thank you for joining us to discuss our fourth quarter 2018 results. This morning, we reported strong fourth quarter results driven by record market share across our core products and record Open Trading activity. Overall, trading volume of $442 billion, was up 24% compared to Q4 2017. Our estimated US high-grade market share reached a record of 19.3%, up from 17.6% last year. Estimated high-yield market share shows especially strong growth and reached a record of 10.9%, up from 6.3% in the same period last year.
Open Trading had a record quarter with volume of $118 billion, up 96% year-over-year.
Fourth quarter revenue of $112 million was up 14% compared to Q4 2017. Operating income for the quarter was $54 million and diluted EPS was up 38% to $1.21.
According to Greenwich Associates' Institutional Investor Survey published in the fourth quarter, MarketAxess maintained its leading position in US credit electronic trading with an estimated 85% market share.
Marketaxess Holding Inc (MKTX) Q4 2018 Earnings Conference Call Transcript -- The Motley Fool