Methods, compositions and kits are disclosed. The compositions are light emitting and comprise a polymeric matrix having dissolved therein a photoactive compound. The composition has the characteristic that, after activation of the photoactive compound, the rate of decrease in the intensity of light emission at any time during a 20-fold decrease in the intensity is proportional to the intensity of the light emission. In one embodiment the polymeric matrix is comprised of particles of about 20 nm to about 100 .mu.m in diameter to which is bound a specific binding pair member. The particles generally comprise a polymeric matrix having dissolved therein about 1 to about 20% by weight of a dopant. The compositions may be used in methods for determining an analyte. A combination is provided comprising (1) a medium suspected of containing the analyte, (2) and the aforementioned composition. The photoactive substance is activated and the effect of the activating on the optical properties of the combination is detected. The presence and amount of the effect is related to the presence and amount of the analyte in the medium. Also disclosed are kits for use in an assay.