In this work we studied the optical and morphological properties of thin films of lead sulfide (PbS) deposited chemically from a solution of lead nitrate, Pb(NO3)(2), sodium hydroxide, NaOH, and thiourea, SC(NH2)(2) diluted in water. The photoconductivity of the films was activated by heat treatments at 353, 423, 473 and 573 K in oxygen residual pressures of 1, 10(-1) and 10(-2) Pa. The results show that two-layer films treated at 473 K at 10(-2) Pa oxygen partial pressure had the largest photosensitivity about 120 times more than another films.