Oxide materials with a perovskite-derived layered structure often show better photocatalytic performance compared to nonlayered materials of similar composition. Here we investigate the catalytic activity for the oxygen evolution reaction (OER) on surfaces of the layered Carpy−Galy oxides Sr 2 Ta 2 O 7 and Sr 2 Nb 2 O 7 . We find that the (010) surface parallel to the layering shear plane is not relevant for photocatalysis. The (100) surface perpendicular to this plane, on the other hand, can catalyze the OER, at least for Sr 2 Ta 2 O 7 , whereas Sr 2 Nb 2 O 7 is inactive for the OER without a cocatalyst, both in agreement with experiment. The OER activity of Sr 2 Ta 2 O 7 ( 100) is, however, only retained while the surface does not assume a reconstructed stepped structure via Sr dissolution which we predict to be thermodynamically favored. Engineering Sr 2 Ta 2 O 7 into long-lasting photoelectrodes thus has to focus on exposing (100) facets as well as preventing Sr dissolution, for example, via a high Sr concentration in the electrolyte.