We present results of a detailed scanning tunneling microscopy (STM) study of submonolayer nucleation and growth of two-dimensional Au islands during deposition on clean versus S-precovered Ru(0001), at room temperature. In the presence of tiny amounts of S, a dramatic increase in the mean island densities is observed, corresponding to a decrease in the effective mobility of Au on Ru(0001). As imaged also by STM, the structure of the S adlayer changes during Au deposition, from a dilute lattice-gas, at low Au coverages, to an increasingly denser sequence of ordered superstructure p(2 × 2), ( 3 × 3 - R 30° ) and c(2 × 4) domains, with increasing Au coverage. These findings suggest that the deposited Au atoms displace S adatoms, compressing S domains, a phenomenon consistent with a net repulsive interaction between neighboring Au and S mediated by the Ru substrate.