New measurements of the radioactive tracer diffusion coefficients of selenium-75 and lead-210 in PbSe, obtained using a vapor source diffusion technique, are reported. These measurements yield significantly lower values of the diffusion coefficients for both species than those previously reported by others. For selenium-saturated PbSe, DSe*=8.13×10-4exp(-1.33 eV/k T) and DPb*=9.81×10-4exp(-1.67 eV/k T), while for lead-saturated PbSe, DSe*=0.553 exp(-2.22 eV/k T) and DPb*=3.74×10-2exp(-2.05 eV/k T). An analysis of these results indicates that selenium atoms diffuse by means of interstitial defects in selenium-rich material and most probably through selenium vacancies in lead-rich material, and that lead atoms diffuse through lead vacancies in selenium-rich material and most probably through lead interstitials in lead-rich material. The tracer data are used in conjunction with a theoretical model of interdiffusion to predict values of the p-n junction diffusion coefficients in PbSe which are in excellent agreement with published experimental results. A similar comparison of the tracer data with n-p junction diffusion data shows good qualitative agreement. Quantitative discrepancies are observed which are explained by the theoretical model.