Vapor-phase epitaxial ZnS:N,Ag/GaAs layers co-doped with Cl or In were investigated by Hall-effect and photoluminesnce measurements. Hall-effect measurements at room temperature revealed free hole concentrations and mobility values to be (1-7) x 10(17) cm(-3) and 60-70cm(2)/Vs for ZnS:N, Ag, Cl layers, and to be (0.6-1.4)x 10(19)cm(-3) and 10-25 cm(2)/V s for ZnS : N, Ag. In layers, respectively. Almost temperature independent free hole concentrations of these layers indicate the formation of impurity band. Concentrations of electrically active acceptor, donor, and neutral impurities were estimated from the temperature dependences of mobility values and hole concentrations. On the basis of the theory of ion pairing, the nearest-neighbor nitrogen-donor (In or Cl) pairs are suggested as the origin of neutral impurity centers contributing to the mobility. Appearance of the "blue-Ag emission" and low-resistive p-type conduction were compatible only for ZnS:N.Ag,In layers.