Photoconduction in heterogeneous thin films consisting of a porous nanocrystalline TiO2matrix and internally adsorbed quantum size PbS clusters is reported. The TiO2serves to establish electrical contact to the PbS clusters, and the PbS cluster size is chosen so that photogenerated excess electrons are directly injected from the PbS to the TiO2. The films exhibit strong photoconductance in the visible region, indicating photoelectric sensitization of the TiO2by the PbS clusters. Comparison of the absorption and photocurrent spectra shows that only PbS clusters of sizes smaller than ∼25 A˚ contribute to the sensitization. For larger clusters the band alignment at the TiO2/PbS interface appears to be unfavorable for carrier transfer. Further improvement in the photoresponse will have to involve optimization of the transport properties in the nanocrystalline TiO2. ©1995 American Institute of Physics.