Diffusion theory is often used to model the transport of light within tissue. It can be used to calculate the light fluence rate in tissue, for example, during photodynamic therapy, or to measure the absorption and scattering properties of tissue. For both of these applications, the influence of the interface between the tissue and the exterior medium on the fluence rate inside the tissue must be known in order to make accurate calculations. We present an experimental investigation of the effect of the refractive index mismatch at the tissue interface on the internal light fluence rate and on the spatially resolved diffuse reflectance as the boundary conditions of the tissue/external medium are changed. The effects of changing the relative refractive index at the boundary are compared to predictions of diffusion theory. The effect of the refractive index mismatch is predicted correctly by diffusion theory.