Photoelectrochemical cells that convert water and/or carbon-dioxide to hydrogen and gaseous hydrocarbons generate bubbles at the surfaces responsible for light absorption and electrocatalysis. Reduced power conversion efficiencies are often ascribed to the presence of large bubbles, but it can be difficult to decouple the optical and electrochemical effects of gas evolution. We have evaluated the effects of gas evolution at macroscopic device areas with microscale features in relevant orientations with respect to the gravitational force vector. Microstructured Si surfaces decorated with thin films of Pt and TiO2/Ni were shown to reduce the departure diameter of H2 and O2 gas bubbles, respectively, relative to a planar surface. The gas coverage was strongly sensitive to the diameter and pitch of microwire arrays. Redox active tracer molecules were used to quantify mass-transfer coefficients in growing bubble films in 0.50 M H2SO4(aq) and 1.0 M KOH (aq). Ray-tracing simulations predicted that bubbles much smaller than the device area do not scatter sunlight away from the absorber surface and were supported direct measurements of the external quantum yield. The results indicate that microstructured electrodes operate acceptably for unassisted solar-driven water splitting regardless of orientation with respect to gravity and that bubbles provide a net benefit to upward-facing photoanodes.