ABSTRACTA fundamental understanding of the expansion kinetics of expandable polystyrene (EPS) is crucial for the design and optimization of processes for EPS‐filled syntactic foams. In this study, a general formulation was developed to model EPS expansion. A semi‐analytical solution was obtained on the basis of the case of a single bubble expansion in an infinite matrix. The dimensionless bubble radius and pressure were defined and found to be exponential functions of the dimensionless expansion time. The characteristic bubble expansion time was able to characterize the timescale of the expansion process. The semi‐analytical solution could qualitatively predict the radial expansion of the EPS microsphere observed in a real‐time experiment. To obtain an accurate prediction, a numerical solution was obtained to the model that coupled the nucleation and expansion of multiple bubbles in a finite matrix at various temperatures. The results show that the numerical solution was able to quantitatively predict the radial expansion of EPS. A parameter sensitivity study was performed to examine the effect of each parameter over the expansion process. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43886.