The evolution of clean, In-terminated InP(100)-(2x4) surfaces is investigated by synchrotron-radiation-excited photoemission spectroscopy as a function of annealing temperature. As-prepared InP(100)-(2x4) surfaces are found to be free of metallic indium, and the 4d core level shows two clear surface components. A third, indium-cluster-related component appears after annealing above 360+/-10 degreesC, due to phosphorous desorption, and is accompanied by a corresponding reduction in intensity in the In-P surface component. Further annealing leads to a decrease in binding energy of the indium-cluster-related peak due to increased metallicity and hence core-hole screening in the clusters. The increasingly metallic nature of the indium clusters is also revealed by the appearance and growth of a Fermi edge in valence-band spectra.