It is often assumed that the photoresponse or incident photon-to-current conversion efficiency (IPCE) spectrum of a sensitized semiconductor electrode is directly correlated with the amount of sensitizing species present on the semiconductor surface. In reality, the various forms of adsorbed species, such as dye aggregates or dye molecules bound to different adsorption sites, such as terrace edges, can have significantly different electron injection yields and carrier recombination rates. To provide information about the amounts of the various adsorbed dye species and their effectiveness as sensitizers, we report the simultaneous acquisition of IPCE and attenuated total reflectance (ATR) UV-vis spectra for a thiacyanine dye bound to a single-crystal oxide semiconductor electrode surface. ZnO single crystals were fashioned into internal-reflection elements to act both as a waveguide for the internally reflected probe beam for UV-vis spectra and as the substrate for dye sensitization using dyes with distinct spectral signatures for monomers and aggregates. Strong agreement was observed between the quantum efficiency and ATR UV-vis spectra, suggesting that, under the conditions employed, both monomers and aggregates of the dye studied generate photocurrent with the same efficiency.