Processes that generate decay of polarization in a cross-linked, nonlinear optical polymer have been studied, and their relationship to a corresponding change in second-order nonlinearity (chi(2)) has been determined. Measurements of second-harmonic generation, thermally stimulated discharge current, and UV-visible absorbance have been employed to identify polarization decay resulting from relaxation of oriented molecular dipoles, migration of space charge, and chemical degradation of chromophores. The effect of process parameters on chi(2) stability in these materials is observed and explained.