Metal−semiconductor nanohybrids (or heterostructures), such as Au−CdS, have become an important class of materials because of their role in photochemical hydrogen production and in other catalytic reactions. Here we report the results of photophysical studies of the interactions of these particles with ATTO dyes (ATTO 590 and 655), which are used as fluorescent probes in a wide range of spectroscopic techniques, most notably super-resolution microscopies. The most important feature of the Au−CdS particles is that they provide the possibility of selective excitation at either their CdS or their Au domains, which absorb preferentially at wavelengths shorter or longer than 500 nm, respectively, thus making possible an excited-state charge transfer reaction from ATTO. Fluorescence quenching of ATTO is dominated by charge transfer to either the CdS domain (λex = 400 nm) or the Au domain (λex = 570 nm). This quenching is quantified by steady-state and time-resolved absorption and fluorescence measurements, and its assignment is confirmed by electrochemical measurements. The results indicate that the ATTO dyes are sensitive and useful probes for measuring the photocatalytic activity of nanoparticles. Characterizing the nonradiative processes of the ATTO dyes in the presence of these catalytically active particles provides a means of gauging their utility in the wide range of spectroscopies in which they are employed.