Spectral response measurements have been made on metal‐semiconductor junction diodes of single‐crystalp‐type zinc phosphide (Zn3P2) and Al, Mg, and Be. Two different experimental configurations were used and the minority‐carrier diffusion lengths calculated using a simple theoretical treatment of the data. In one experimental mode, the light is incident on the surface of the semiconductor opposite the metal junction. The spectral response is peaked and a lower limit on the diffusion length can be calculated from the peak height and sample thickness without detailed knowledge of the optical properties of the semiconductor. In the other mode, light is incident through the metal layer and the diffusion length is calculated by analyzing the spectral response function using known values of the optical absorption coefficient. On samples for which both modes were used, the results are in good agreement. The diffusion lengths measured are in the range of 5–10 &mgr;m.