The first application of perturbed-angular-correlation (PAC) spectroscopy to study the incorporation and diffusion of In atoms on GaAs surfaces is reported here. A PAC-MBE system has been established to perform in situ PAC measurements on surfaces grown by molecular beam epitaxy. To define the surface conditions that the PAC experiments are based on, scanning tunneling microscopy is used to characterize the topography of similar surfaces grown under identical conditions. To perform the PAC surface experiments, In-111-Cd-111 radioactive probe nuclei are deposited onto the surface to be studied in trace quantities (10(-4) ML). The measured physical quantity is the electric-field-gradient tensor at the probe site. Preliminary results show that In atoms are incorporated into several types of sites on (001) and (111)B surfaces. These surface sites are characterized by well-defined electric-field gradients. These results show that PAC spectroscopy is sensitive to the local atomic arrangement of surface atoms. Moreover, annealing experiments show that changes in electric-field gradients associated with surface-site conversion characterize surface processes on GaAs such as diffusion, desorption, and adsorption.