The growth rate in atomic layer deposition (ALD) or epitaxy (ALE) is usually saturated to a constant level when appropriate operation conditions are attained. The layer thickness deposited in one cycle is limited by adsorption. A simple molecular description for chemisorption and surface reactions is suggested and shows that both reaction steps have an effect on the growth rate. A model is developed for the calculation of the growth rate from the geometry of the reactant molecules and the density of the adsorption sites on the surface. The results are compared with experimental values of binary oxide, sulphide and fluoride thin films. In most cases, a surface configuration is found that gives the observed growth rate.