Poly(1-(trimethylsilyl)-1-propyne), PMSP, is known to absorb large amounts of fixed gases such as carbon dioxide and methane, presumably due to the inherently large free volume of PMSP. Exposure to carbon dioxide at pressures as high as 1000 psia apparently increases chain mobility, allowing for relaxation of excess volume in the polymer matrix induced by toluene solution casting. These volumetric relaxations, determined with a three-dimensional dilatometer, appear in the form of a time-dependent decrease in the overall polymer volume upon depressurization of the carbon dioxide. Repeated exposures to carbon dioxide show no further decrease in sorption capacity and essentially no further decrease in overall volume. PMSP did not exhibit the typical conditioning behavior in which volume and sorption capacity increase upon subsequent repressurization following desorption. This absence of "normal" conditioning behavior was attributed to residual "preconditioning" by the toluene casting solvent. The PMSP stiff chain backbone relaxes so slowly after casting that the polymer retains massive amounts of excess volume even after initial exposure to plasticizing carbon dioxide.