Solution-gas drive in reservoirs containing heavy and viscous oil is not well understood. The full-length paper develops a mechanistic population-balance model for describing the bubble-nucleation and -growth process. Appropriate rate equations are derived for instantaneous-nucleation (IN) and progressive-nucleation (PN) theories. The PN model matches the experiments somewhat better but is more computationally demanding. The population-balance description of both models does not require a critical supersaturation to be exceeded before bubble-nucleation and -growth onset. Liberation of gas from solution at the thermodynamic bubblepoint, and bubblegrowth equations presented describe gas-phase kinetics and pressure response of the systems examined.