We have characterized the concentration and time dependences of the attachment of colloidal CdSe quantum dots (QDs) to 16-mercaptohexadanoic acid (MHDA)-functionalized nanocrystalline TiO2 thin films. The amount of QDs and the extent of their agglomeration on MHDA-functionalized TiO2 films were characterized by transmission- and reflectance-mode UV/vis absorption spectroscopy and scanning electron microscopy. Optically transparent films with spatially homogeneous coloration and minimal agglomeration of QDs were prepared from 2.2 and 5.0 mu M toluene dispersions of QDs at short reaction times (<5 h). In contrast, prolonged exposure of MHDA-functionalized TiO2 films to 22 mu M dispersions of QDs yielded relatively opaque QD-functionalized films with spatially inhomogeneous coloration and substantial agglomeration of QDs. Agglomeration of QDs decreased the absorbed photon-to-current efficiencies of QD-sensitized solar cells (QDSSCs) by almost 3-fold. These results highlight the profound influence of agglomeration on the optical properties and interfacial electron-transfer reactivity of QD-functionalized TiO2 films prepared by in situ linker-assisted assembly as well as the photoelectrochemical performance of QDSSCs incorporating such films.