Two-phase thin-film composites consisting of CdxPb1 – xS (0.007 ≤ x ≤ 0.068) substitutional solid solutions with a B1 cubic structure (sp. gr. ) and amorphous cadmium sulfide (CdS) have been produced by chemical deposition. The surface topography of the films has been studied by atomic force microscopy, and the results have been used to calculate surface microtopography parameters. A correlation has been found between the composition and functional properties of thin CdS–PbS films. The sensitivity of CdxPb1 – xS/CdS two-phase films to ammonia (NH3) in air has been assessed for the first time. The room-temperature detection limit for ammonia has been determined to be 10 ppm (6.22 mg/m3).