Solubilities of carbon dioxide and nitrogen in molten polypropylene (PP) and high-density polyethylene (HDPE) were measured at temperatures 433.2, 453.2, and 473.2 K and pressures up to 17 MPa. The solubilities increased almost linearly with pressure. While the solubilities of carbon dioxide decreased with increasing temperature, those of nitrogen increased in the temperature range examined. The solubility of nitrogen in glassy polystyrene (PS) was measured at 313.2, 333.2, and 353.2 K and pressures up to 17 MPa. Temperature dependence on the solubility in this system was distinctive, where minimal solubility of 310 cm³(STP)/(kg MPa) was observed around 350 K, by taking into account of our previous data above 373.2 K. Henry's constants, K_P, and their temperature dependence were determined from the experimental data. A linear relationship between ln(1/K_P) and (T_C/T)² was obtained for each system. Diffusion coefficients of gases in molten polymers were determined for the nitrogen+PP, nitrogen+HDPE, and carbon dioxide+HDPE systems at 453.2 K. The diffusion coefficients showed weak concentration dependence and had an order of magnitude of 10⁻⁹ m²/s.