The minority carrier diffusion lengths on both sides of Cu2S&sngbnd;CdS heterojunctions have been measured directly by light microprobe techniques. In approximately 1 &OHgr; cm CdS the hole diffusion length was found to be between 3×10−4and 7×10−4cm. The high‐conductivity Cu2S formed on the CdS by a chemical dip was found to have electron diffusion lengths ranging from ≤1×10−5to 4×10−5cm. Light‐induced breakdown was observed at small reverse bias for photon energies greater than the CdS bandgap. The reverse breakdown was correlated with visible defects on the junctions where photocurrent gains as large as 2000 were observed. A model for the breakdown is proposed in which Zener breakdown at high‐field points along the junction is modulated by photoinjected holes.