Solution gas drive in reservoirs containing heavy and viscous oil is not well understood. This paper develops a mechanistic population balance model for describing the process of bubble nucleation and growth. The model is applied to both light and viscous oils. The primary modeling concept is a continuum bubble population balance. Appropriate rate equations are derived for two theories of bubble nucleation described in the literature—instantaneous nucleation (IN) and progressive nucleation (PN). The results of simulations for the IN and PN models are compared to experimental data reported elsewhere for light oil and to new data for viscous oils. Model parameters are all physically based. Within the IN model, the number density of bubbles must be specified while the PN model requires the cavity size distribution of the porous medium as input. The PN model matches the experiments somewhat better, but is more demanding computationally. Interestingly, the population balance description of either model does not require a critical supersaturation to be exceeded before the onset of bubble nucleation and growth. Supersaturation is the difference between the equilibrium and dynamic liquid pressure of a system. Liberation of gas from solution at the thermodynamic bubble point and the bubble growth equations presented here well describe the kinetics of the gas phase and pressure response of the systems examined.