We have imaged hydrogen on Si~001! at low coverages in a variable-temperature STM from 300 K up to 700 K. Individual hydrogen atoms were imaged which became mobile at around 570 K. The observed rate of hopping along the dimer rows was consistent with an activation energy of 1.68 6 0.15 eV. Motion across dimer rows was rarely observed, even at the higher temperatures. The diffusion barrier for motion along the dimer rows has been calculated using tight-binding and density-functional theory in the generalized gradient approximation ~GGA!. The calculated barrier is 1.65 eV from tight binding and 1.51 eV from GGA. @S0163-1829~96!07543-1#