Ex situaqueous (NH4)2S treated sulfur-passivated InP substrates have been studied using ultrahigh vacuum scanning tunnelling microscopy (STM) and low-energy electron diffraction (LEED). The morphology of the passivated surface was imaged after a mild sample annealing. The STM images of a surface exhibiting a good 1×1 LEED pattern show that the top layer of the sulfur-passivated surface is poorly ordered. A surface bilayer atomic step has been observed to be common on sulfur-passivated surfaces. The magnitude of the surface roughness for the passivated surfaces lies between 10 Å and 25 Å; this is much smaller than the roughness of InP(100) substrates prepared using previously published methods. After annealing the sample at∼420 °C, a (2×1) LEED pattern with split half-integer spots has been observed. The associated STM images show that these split half-integer diffraction beams correspond to regularly spaced domains with a width of∼20–30 Å in the [011̄] direction. The surface roughness increases with annealing temperature; the surface corresponding to the best 2×1 LEED symmetry (annealing at∼420 °C) has a roughness double that of the 1×1 phase.