Comparison of x-ray fluorescence and electron-yield surface extended x-ray absorption fine struction (EXAFS) above the S K-edge for c(22)S on Ni(100) reveals an order of magnitude higher sensitivity for the former technique due to a much lower background. The higher sensitivity of soft x-ray fluorescence EXAFS opens the possibility of structural studies on very dilute atomic species on surfaces (0.1 monolayer) or in the bulk (1019atoms/cm3) of a different material. An EXAFS study on the local structure of S impurities in GaAs is presented as an example of such studies. We derive the local geometry around the S atoms finding two competing configurations of approximately equal population: (1) substitutional S on an As site (2) a complex formed by S on an As site and an As vacancy on the second shell with a S-first neighbor distance relaxation of 0.140.04. The two configurations explain the disparity between implanted S concentration and net electrical activity. © IOP Publishing Ltd.