Recent work on the III–V surfaces has indicated that adsorbate coverages ≲0.01 monolayers can significantly influence the surface electronic structure. The use of secondary ion mass spectrometry (SIMS) to study sorption phenomena on (110) UHV‐cleaved surfaces of InP at levels below 0.01 monolayers is reported here. The analyses reveal that significant quantities of adsorbates can accrue on a surface cleaved and stored in UHV, with a very rapid initial contamination of the freshly cleaved surface. For ∠0.1 L exposure to UHV total contaminant coverages ≳10−3monolayers are observed. The surface chemistry appears complex, with phosphorus oxides being the predominant species. The oxidation is apparently due to a dissociation/adsorption reaction with CO, with the additional possibility of adsorption of volatile phosphorus oxides initially desorbed from the sides of the cleaved boule. The above sorption phenomena are believed to be associated with a relatively small number of highly active surface sites, for example at cleavage steps. Further exposure to UHV up to ∠ 10 L results in partial saturation in the surface contamination coverage. Exposure of this surface to O2(4×105L) leads to further oxidation and possible chemisorption of O2onto P dangling bonds. The SIMS spectra contain evidence for an increasing level of P–In bond breaking with UHV and O2exposure, and this is associated with contamination‐induced surface disorder.