A model of reactive ion-exchange diffusion in glassy materials (glasses and glass-ceramics) has been developed. The model can be used to simulate the processes of ion-exchange-induced decrystallization of glass-ceramics and ion-exchange-induced glass crystallization. Analysis of the model in dimensionless form resulted in determining a set of dimensionless complexes comprising dimensional parameters of the ion exchange system (diffusivity, rate constant of grain dissolution, initial size and composition of the crystalline grains, etc.). Numerical values of the complexes enable one, not solving the problem, to predict the composition and structure of glass-ceramic subsurface layers produced in a certain glassy material and under given ion-exchange conditions. It was shown that the developed model correctly describes experimental data on ion-exchange-induced glass-ceramics decrystallization. It appears that the model can be used in developing a new technology of optical glass-ceramics for photonic applications.