The analysis of effective solubility products of lead and tin(II) sulfides at the «PbS solid film- acetate SnCl 2 aqueous solution» interface, along with analysis of a change in the Gibbs free energy of PbS → SnS ion-exchange transformation has been performed and confirmed the possibility of formation of Pb 1- x Sn x S substitutional solid solutions in the considered system. The results of scanning electron microscopy and energy-dispersive analyses have revealed changes in the elemental composition and surface morphology of tin-modified PbS layers in relation to duration of contact between a film and an acetate tin(II) chloride aqueous solution. For the first time, tin-containing films based on lead sulfide with the tin content up to 33.3 at % including a phase of Pb 1- x Sn x S solid solutions have been obtained by ion-exchange substitution via incubation of thin polycrystalline PbS films in tin(II) chloride aqueous solutions at 368 K.