Optical in situ methods are becoming of increasing importance for studies of the mechanisms of growth and for real-time control of surface processes involved in epitaxial growth. We present data from studies, using the reflectance-difference (RD) method, of different processes related to epitaxial growth of GaAs (001) from arsine (AsH3) and from triethylgallium (TEG). RD studies make it possible to follow how the near-equilibrium, Ga-rich, configuration of the GaAs (001) surface changes with temperature and we show how the transformation to this state can be followed from As-saturated as well as from Ga-saturated surfaces. From the initial slope of the RD-transient, corresponding to the AsH3 reaction with a Ga-stabilized surface, the temperature dependence of the catalytic effect of this surface on the decomposition of arsine is measured. Of special significance for an atomic layer epitaxial process (ALE) is the demonstration of a self-limiting reaction of TEG. This process can be followed as the surface modifies itself from a state obtained by exposure to excess TEG to the near-equilibrium Ga-terminated reconstruction. Finally, we demonstrate how pulsing sequences for the realization of ALE can be followed and controlled in real time using RD as a diagnostic tool.