We have deduced the spatial distribution of defects in light‐soaked hydrogenated amorphous silicon (a‐Si:H) from the thickness dependence of the areal defect density using a large number of film thicknesses, ranging from 0.05 to 8.7 &mgr;m. The light soaking was done with strong white light generated by a Xe lamp with an infrared‐cut filter and the defects were measured using electron spin resonance. The distribution of defects is found to be highly nonuniform and has an inverse power‐law formNv(x)=Ax−&agr;, whereNv(x) is the defect density at depthxmeasured from the surface, andAand &agr; (≊0.6) are constants and depend on the light‐soaking time. Our results show unambiguously that the Staebler–Wronski effect is a bulk effect, however the regions close to the surface are affected much more by light soaking than the regions deep in the bulk of the sample. ©1995 American Institute of Physics.