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Summary form only given. Fluorescent dyes used in dynamic measurements of cellular calcium signals have been found to bind to endogenous proteins. When these proteins are immobile or nearly immobile, dye binding hinders drastically the diffusion of the dye, and therefore, the diffusion of calcium. Beginning with a full model of calcium buffering and dye binding, we derive a simplified model based on the assumption of rapid local equilibrium. The dye is assumed to bind to immobile, endogenous proteins that hinder its diffusion, yet may still bind Ca2+ and fluoresce nominally even when bound. This model is then used to investigate the nature of both calcium and dye diffusion. We also explore conditions under which the simplified model remains valid by comparing it to the full model.