The design of gas-liquid separators is commonly based on the American Petroleum Institute (API) 12J guidelines. Although these guidelines have been useful, they may fail to provide sufficient time required for complete gas-liquid separation and may lead to problems, such as gas carry-under (GCU), in some cases. Therefore, an accurate understanding of gas evolution rates (volumetric mass transfer coefficient) from supersaturated solutions is critical. The objective of this work was to elucidate the influence of pressure and temperature on the rate of gas evolution from a supersaturated model oil (Exxsol D-110) and three crude oils (crude A, crude B, and crude C) of varying viscosities. The initial pressure was varied up to 10.45 MPa, keeping the temperature constant at 298.15 K. The increase in the initial saturation pressure showed that the volumetric mass transfer coefficient (gas evolution) of Exxsol D-110 and crude B (having the same viscosity) exhibited different trends. In contrast, crude A and crude C with different viscosities exhibited similar volumetric mass transfer coefficients. The effect of temperature was determined by varying the liquid temperature from 288.15 to 348.15 K at a constant initial saturation pressure of 3.45 MPa. The volumetric mass transfer coefficient increased with an increase in temperature. At similar viscosities, crude oils exhibited different volumetric mass transfer coefficients. On the basis of our experimental data, it was observed that the viscosity of the oils was not the only factor that affected gas evolution. For all cases considered in this work, the time required to evolve 50% of the gas was higher than the time estimated by the API 12J guidelines.