A compensated layer with a very low net ionized impurity density was found in ZnSe p-n homojunctions. We studied these devices by measuring their capacitance-voltage measurements and with secondary ion mass spectroscopy. The p-n junctions were prepared with Ga diffusion in p-type ZnSe bulk single-crystal substrates or in p-type liquid-phase epitaxial layers. The bulk crystals as well as the p-type epitaxial layers, were grown by the temperature difference method under controlled vapor pressure using Se solvent with Na2S as a dopant. The Ga diffusion profile has a deep tail region with low Ga concentration. The tailing is greatly intensified for samples grown at low Se vapor pressures or without application of Se vapor pressure, although these samples exhibit a blue luminescence. It was found that a thick compensated layer, which lowers the injection efficiency, appeared as a result of this tailing. At higher Se vapor pressures, the compensated layer is not as thick, but the luminescence from self-activated centers becomes dominant. The compensated layer, as well as Ga tailing region almost disappears when the diffusion time is short even without application of Se vapor pressure.