AgInS2 and AgInS2:Sn semiconductors are currently interesting because of their potential applications like solar cells. Earlier results show that electrical and optical properties of AgInS2 appear to be dominated by donor-like intrinsic defects because it usually displays n-type conductivity. Chalcopyrite AgInS2 and AgInS2: Sn thin films were prepared by spray pyrolysis. It was noted that the Sn-doping produces no appreciable effect on the structural neither the optical properties. The remarkable difference between undoped and some Sn-doped samples lies in the conductivity type. Photoluminescence (PL) studies reveal that Sn reduces sulphur vacancies: electron paramagnetic resonance (EPR) studies in Sn-doped samples, show two superimposed signals close to the g-2 region. One of them was assigned to punctual defects in the lattice due to substitution of Sn instead of In or S sites. This promotes a conductivity increase in this material; the second one shows a hyperfine structure which implies a nuclear spin l=9/2 assigned to In in the lattice. Both signals were used to determine relative Sn quantities which substituting In or S determining the conductivity type. Published by Elsevier B.V.