The development of expansion injection molding as new technology for producing thin plastic parts has raised questions about polymer melt behavior in the process of rapid high pressure compression and expansion. To investigate those phenomena, the new in-line injection molding machine mounted measurement system has been developed. Measuring nozzle equipped with hydraulically driven closing bolts and contact fast response pressure and infrared temperature sensors enabled us to measure the compression and expansion of polymer melts at different starting conditions. Results presented for high impact polystyrene and polyamide show that polymer melt temperature rise is linearly dependent on compression pressure as well as it is dependent on compression speed. Comparable effect of temperature fall has been recorded during the polymer expansion, resulting in no noticeable loss of dissipation energy during this reversible process. Measurements have been compared with modified 2-domain Tait equation of state. Curve fitting analysis to manufacturer's material data show that measurements fit well with the model, even at very high compression speeds.