The solubility, C*, and volumetric mass transfer coefficient, k(L)a, values for hydrogen and ethylene in liquid propylene were obtained in a 41 surface-aeration agitated reactor operating under pressures between 11 and 55 bar, temperatures from 297 to 33 3 K, and mixing speeds of 13.3 20.0 Hz. The pressure-time profile of the hydrogen propylene system exhibited an anomalous behavior due to the vaporization of liquid propylene into the gas phase. The equilibrium gas solubilities were calculated using a modified Peng-Robinson equation of state (PREOS) and the mass transfer coefficients were determined using the transient physical gas absorption technique. A calculation procedure for determining the equilibrium composition and mass transfer coefficients for hydrogen in liquid propylene which accounted for the anomalous behavior of this system was developed. The equilibrium vapor liquid mole fractions obtained using this procedure compared favorably with available literature values. The C* values were found to increase with the partial pressure of the solute gas. The k(L)a values increased with mixing speed for both gases. The solubilities of ethylene in liquid propylene were found to be higher than those of hydrogen, whereas the mass transfer coefficients for hydrogen were appreciably higher than those of ethylene. An empirical correlation which predicted k(L)a values for hydrogen and ethylene gases in liquid propylene in a surface-aeration reactor with an accuracy of +/- 30% was developed.