Anisotropic chemical etchants are used widely to fabricate silicon microstructures; however, the roles of surface heterogeneity and localised chemical effects during the etch process are still unclear. This investigation has employed photoelectrochemical microscopy to resolve spatially the reactivity of p-Si [100] and [111] during etching and passivation in 2.0 M KOH. Potentiodynamic photocurrent measurements demonstrate that the interfaces follow the ideal Gartner response until an anodic oxide is formed. Examination of the potential-dependent transient photocurrent response indicates that formation of an oxide inhibits charge carrier transfer and pins the Fermi level. Furthermore, the growth and dissolution of the oxide are reversible over a time scale of minutes. In situ optical and subsequent atomic force microscopies show that during the etching process surface roughening occurs preferentially at particular sites, the density of which is greater at [100] oriented samples, and corresponds to regions at which pyramids are formed. Photoelectrochemical microscopy results provide spatial information on the reactivity of interfaces, Both the [111] and [100] orientations exhibit responses which are attributed to lattice defects at the semiconductor surface. These act as recombination centres but do not manifest themselves in the final etch morphology. Examination of the influence of the electrode potential and effect of pre-passivation indicates that the oxide formed is uniform and does not exhibit pinholes as in fluoride-containing media. However, real-time photocurrent imaging of the oxide removal process resulted in quite different behaviour, and an additional photoimage contrast was observed which differed considerably between the two crystal orientations. The origin of this heterogeneity is attributed to the different dissolution kinetics of the oxides formed.