A high pressure, efficient and power dense air compressor/expander is a critical element in an isothermal compressed air energy storage (ICAES) system. Heat transfer is often the limiting factor in realizing efficient and power dense compression and expansion processes. Liquid piston compressor/expanders with porous media inserts have been proposed, in which the porous media serves as heat exchangers. While this concept has been studied through modeling and simulation, it has only been validated experimentally at low pressures (10 bar). This paper studies experimentally the effect of porous media in a high pressure (7–210 bar) liquid piston air compressor/expander, which is the proposed pressure for the ICAES system. Cases with the porous media uniformly distributed and non-uniformly distributed in the chamber are studied with various compression and expansion rates. Results show that at 93% efficiency, the uniformly distributed 2.5 mm interrupted plate porous medium increases power density by 10 times in compression and 20 times in expansion; or at the same power, efficiency is increased by 13% in compression and 23% in expansion. Moreover, the porous medium, if deployed at the top of the chamber, is shown to be more effective than if deployed at the bottom. The results indicate that the added surface area provides the dominant benefits but the porous media also increase the heat transfer coefficient at the same efficiency regime. These results are consistent with and extend the findings from previous low pressure experiments. [ABSTRACT FROM AUTHOR]