Macroporous GaP photoelectrodes with wall thicknesses of approximately 500 nm and pore depths ranging from 0 to 45 μm have been prepared from nondegenerately doped single-crystalline n-GaP(100) with a minority-carrier diffusion length of only 110 nm. The photoelectrochemical behaviors of planar and macroporous photoelectrodes have been assessed in nonaqueous regenerative photoelectrochemical cells operated under potentiostatic control and employing dry acetonitrile containing ferrocene/ferrocenium. Enhancements in the short-circuit photocurrents tracked increases in internal quantum yield measurements at long wavelengths for macroporous n-GaP. The observed photocurrent densities and spectral response measurements were consistent with values expected from GaP/liquid heterojunctions controlled by faradaic charge transfer to an outer-sphere, dissolved redox couple. Lower open-circuit photovoltages were observed with macroporous electrodes with increasing pore depth, consistent with distribution of the photocurrent across the entire macropore/solution interface. Fill factors observed under these conditions did not systematically track pore depth in macroporous photoelectrodes. The noted changes in short-circuit photocurrents, open-circuit photovoltages, and fill factors with increasing porosity resulted in more than an order of magnitude improvement in the photoelectrode efficiency of macroporous n-GaP with 45 μm deep pores at 100 mW cm−2 illumination. The presented data show the level and type of enhancement in energy conversion efficiency that high-aspect-ratio electrode architectures can provide carrier-collection-limited materials such as GaP.