A study was performed to establish the origin and nature of background compensating impurities in undoped ZnSe layers grown by liquid phase epitaxy on ZnSe substrate wafers in a low‐contamination‐level environment. The width of bound exciton lines in low‐temperature photoluminescence spectra was used to define the quality of the material, and the energy of the lines was used to identify these low‐level impurities. The sharpest spectra occurred in layers grown rapidly on a previously grown buffer layer indicating the importance of impurity outdiffusion from the substrate into the growing layer. The sharpness of these bound exciton lines indicates that the total concentration of electrically active impurities (NA+ND) is <1017/cc, an estimate which is confirmed by mass spectroscopy.