A procedure has been developed for applying the surface photovoltage technique to the measurement of the width of the oxygen precipitate-free zone present at the surface of a thermally processed, Czochralski-grown silicon wafer. This procedure was developed through the use of a numerical simulation program which models the experimental determination of an effective diffusion lenght, L0, from surface photovoltage measurements on silicon wafers. The program predicts L0 for a given precipitate-free zone diffusion length and thickness and bulk diffusion length. Results of the simulations show that for bulk diffusion lengths of 2 μ or less and a precipitate-free zone diffusion length greater than the thickness of the zone, W, the W ≡ 2.5L0. Experimental results are presented which support the numerical findings. © 1983.