Alkyl siloxy functionalization increases the solubility of polyhedral oligomeric silsesquioxanes (POSS) in supercritical carbon dioxide over an order of magnitude compared to its counterpart with fluoroalkyl groups. The studied component, octatrimethylsiloxy POSS also exhibits higher solubility than other previously studied POSS types with various alkyl and methacryl groups. The octatrimethylsiloxy POSS-CO2 solid-vapor equilibrium curves have been constructed at temperatures between 308 and 328 K by measuring cloud points. The highest measured solubility of octatrimethylsiloxy POSS was 0.176 wt. fr., corresponding to 0.0083 mol fr., at 13.36 MPa, 328 K, and CO2 density of 0.59 g/cm(3). The solubility versus CO2 density isotherms fall on a master curve up to about 2 x 10(-3) mol fraction, while at higher concentrations, higher solubility was attained with increasing temperature. The experimental data were correlated using various semi-empirical density-based models which provided satisfactory correlations.