Potential-step experiments on yttrium thin films show exponential decay of the current as a function of time. A diffusion model for a thin-layer system relates the time constants to effective diffusion coefficients for hydrogen in the di- and trihydride concentration range. The diffusion coefficient is found to be concentration-dependent. From the temperature dependence of the diffusion coefficients and the optical switching time, activation energies are calculated. The good agreement with literature values supports the model of diffusion-limited kinetics for the optical switching.