Photothermal deflection spectroscopy (PDS) is a well-known thermal wave technique [1,2] which is well suited to qualitatively measuring the optical absorption spectra, in situ, of photoelectrochemical (PEC) electrodes [3]. PDS employs a probe laser beam, propagating parallel to the working electrode (WE) surface, to detect the temperature (refractive index) gradient which exists perpendicular to an electrode being excited with intensity-modulated light. The light absorbed by the WE is normally converted to heat by the efficient nonradiative deexcitation of the non-equilibrium, optically-generated electron-hole pairs. Due to the presence of the temperature/refractive index gradient at the WE/electrolyte interface, the probe beam will undergo a deflection which can be measured with a position-sensitive photodetector; usually only the AC signal component is monitored, using lock-in detection, for signal-to-noise ratio (SNR) enhancement. If the relative spectral content of the exciting beam is maintained constant, the PDS signal is found to be a linear function of the maximum illumination intensity [2]; thus, the PDS technique can be employed as a micro-calorimetric probe to measure the relative degree of heating at the WE/electrolyte interface. When the illuminated WE is DC-biased, heat generation within the electrode becomes relatively more complex, but a model has been developed [4] which quantifies the electrode heating under this condition; in this work, this model has been modified for the situation where a probe laser beam is used to detect the degree of WE heating. Also, following the work of Fujishima et al. [4], a method for determining the PEC photo-current quantum efficiency from PDS signal vs. DC bias voltage plots has been developed.