The loss of sphericity associated with instability of hollow polymeric microspheres in the course of expansion is discussed. The loss of local stability in a thick-walled spherical shell made of a highly elastic incompressible material with respect to axisymmetric perturbations retaining the hollow volume is investigated. The shell is assumed to be loaded by the internal hydrostatic pressure produced by the incompressible liquid absorbed by the hydrophilic core. The stability of equilibrium of a nonlinear early elastic body is conventionally investigated using the Euler static method. This method is based on an analysis of equilibrium of states slightly differing from the given state, with determination of the values of loading parameters under which the existence of nontrivial solutions to the equations of equilibrium with appropriate boundary conditions is possible. The nonspherical shape of the initially spherical particles can thereby result from the instability during expansion.