The initial growth of cobalt on the GaAs(001) c(4 X 4) surface has been studied using scanning tunneling microscopy, soft x-ray photoemission spectroscopy, and low energy electron diffraction. The structure and chemical reactivity of the Co layer are studied as a function of the substrate temperature and Co coverage. At low substrate temperatures (-10 degreesC), Co grows as small randomly distributed islands that start to coalesce for coverages exceeding 2 monolayers. Larger diameter islands are formed at substrate temperatures above 95 degreesC. However, higher growth temperatures result in significant interfacial reactions despite an improved surface morphology. Comparisons are made with CoAs and CoGa films grown directly on GaAs(001) and studied in situ with x-ray photoemission spectroscopy and scanning tunneling microscopy.