Electron diffraction, DTA, and optical absorption studies carried on thin films of PbS-HgS prepared by the solution growth technique developed by us suggest the formation of a uniphase ternary alloy with f.c.c. symmetry. The lattice parameter of the Pb(1-x)Hg(x)S alloy films can be made to increase or decrease with increasing concentrations of Hg depending on the choice of growth conditions. In the case of films where the lattice parameter increases with Hg concentration, alloying of PbS with alpha-HgS takes place, and the optical absorption edge shifts continuously toward higher energies. On the other hand, in the films for which the lattice parameter decreases with increasing Hg concentration, alloying of PbS with beta-HgS takes place, and the optical absorption edge shifts to lower energies. Thus the optical gap of the composite material can be varied from 0.2 eV to 1.2 eV by changing the growth conditions.