A new high-pressure variable-volume optical cell equipped with a sapphire window, a movable piston with a position sensor, and infrared fiber optic probes is described. The apparatus was tested by measuring the cloud points for the system propylene + poly(ethylenepropylene) at 100 and 150-degrees-C, the densities of ethylene and propylene up to 1000 and 600 bar, respectively, and the midinfrared spectra of hexane at 500 bar.