Evaluation and excavation by V Hughes and B Atfield of OA revealed an east–west orientated ditch, from which struck flints of probable Mesolithic or early Neolithic date were recovered, and a north–south orientated ditch. As a result of the evaluation, an area measuring 50 x 30m was investigated. The anticipated ditches proved to date from the Late Iron Age, but a substantial scatter of worked flint, not encountered in the evaluation, was revealed close to the south-east edge of the site. Significantly, the flintwork was assigned to either the Creswellian or Federmesser industries of the late glacial period and was found in an exceptional state of preservation, stratified within sand deposits on the edge of the excavation area. They probably represent the in-situ remains of a Later Upper Palaeolithic campsite. At the time of writing, the lithic assemblage totals c 5500 worked flints, although that number is likely to increase when the remaining bulk sediment samples have been sieved. A preliminary programme of Optically Stimulated Luminescence (OSL) dating on the sediments has produced dates consistent with the late glacial period. The assemblage has excellent potential for further, more detailed work that will include refitting and microwear analysis. A full programme of OSL dating and scientific analysis of the sediment sequence will also help to provide a chronological framework and clarify the environments of deposition and site formation processes. Open-air sites of this period, representing the first recolonisation of the British Isles following the last Glacial Maximum, are exceptionally rare and the Guildford site is, therefore, considered to be of national importance.