Abstract This paper is concerned with the nucleation of gallium arsenide on the (111) face of tungsten. After a brief introduction to the more relevant theories of nucleation, a description is given of the vapour phase growth equipment. The growth is analysed in terms of the variation of saturation nucleation density with reciprocal temperature and time, and results are given for the variation of average nucleus diameter as a function of temperature and time. Experimentally determined values of activation energies are given and the saturation density of nuclei is determined. These values are shown to be quantitatively inexplicable by present nucleation theories.