Time evolutions of O 2p intensity during the first monolayer growth of dry oxides on Si(100)-(2 X 1) surface as obtained by real-time ultraviolet-photoelectron spectroscopy (UPS) shows two types of evolutions: Langmuir-Hishelwood type in the low-T/high-P region and a sigmoid-like type in the high-T/low-P region. Preoxidation experiments demonstrates that the former corresponds to the random-adsorption mode and the latter to the 2D-island growth mode. The obtained time evolutions for the two modes, not only the initial stage but also the one up to one monolayer coverage, are almost perfectly described with the autocatalytic reaction model developed by the authors. The physical background of the model is discussed to show that the coalescence of islands is properly involved in the model, which accounts for its ability.