We investigated the wafer uniformity of the etch attack obtained by reactive ion etching of InP in a CH4/H-2 plasma. We found that this process inherently tends to produce a rather nonuniform distribution of etch rates across InP wafers with the fastest etching occuring at the wafer edge. Nonuniformities of up to 35% are observed. We propose a simple model, which explains the observation by reactants, which originate from nonetched regions, diffuse parallel to the wafer surface, and locally enhance the etch rate. The nonuniformity can be efficiently counteracted by a proper choice of the process pressure or even completely avoided by the introduction of a guard ring. In addition we also found that the arrangement of the masked areas on the wafer influences the etch rate profile.