Deep photoresist gratings, slanted as well as unslanted, were produced holographically in clear Shipley 1400 photoresist. The diffraction efficiencies of these gratings were measured as a function of incident angle for three wavelengths with polarization perpendicular to the plane of incidence. It is shown that the results agree fairly well with those predicted by Kogelnik's two-wave theory, indicating that these relief gratings behave like volume holograms. An explanation in terms of thin and thick gratings is given, and practical conclusions are drawn from these observations.