Arsenic adsorption and exchange with phosphorus on indium phosphide (001) have been studied by scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy. The surface phase diagram as a function of temperature has been obtained. At 285°C, arsenic adsorbs on the InP σ(2×4) surface (P coverage=0.25 ML), forming a disordered (1×4) with double layers of arsenic (As +P coverage ∼1.5 ML). At 330°C, arsenic adsorbs on the σ(2×4), producing a (2×1) structure with a complete monolayer of group-V dimers. By contrast, some phosphorus desorption occurs above 350°C, allowing arsenic to displace the phosphorus in the top few layers and converting the σ(2×4) to the β2 and α2(2×4) reconstructions (As coverage 0.75 and 0.50 ML, respectively). Above 430°C, arsenic exchange with InP yields an InAs (4×2) reconstruction. Quantitative analysis of the x-ray photoemission spectra has revealed that substitution of arsenic for phosphorus is limited to the top two to three surface bilayers.