They use IBM/360[1] mainframes. Bloomberg had a good story [2] some time ago that was re-run last April 17th on some of the history. [1] https://www.washingto

They use IBM/360[1] mainframes. Bloomberg had a good story [2] some time ago that was re-run last April 17th on some of the history. [1] https://www.washingtonexaminer.com/tech-timebomb-the-irs-is-still-living-in-the-1960s https://www.washingtonexaminer.com/tech-timebomb-the-irs-is-... [2] https://www.bloomberg.com/view/articles/2018-04-17/the-irs-computer-system-is-the-oldest-in-the-government https://www.bloomberg.com/view/articles/2018-04-17/the-irs-c...