The solid state diazotation of sulfanilic acid monohydrate (1) on (010) gives island features and craters on the surface. The remaining surface roughness stays low enough to permit depth dependent grazing incidence X-ray diffraction (GID) measurements, but only when the gas applications are slow. The theoretical background of the unprecedented GID measurements is given. GID measurements were undertaken below, at and above the critical angle of incidence to differentiate the processes at the surface and in the crystal bulk. The phase rebuilding and phase transformation processes could be directly detected in the bulk of the crystal supporting the common AFM interpretations which are derived from the formation of surface features in correlation with crystal packing data. The combined consideration of depth resolved in-plane GID and AFM experiments leads to a comprehensive description of the gas-solid process.