A detailed model for the operation of a flow type atomic layer deposition (ALD) reactor is developed and tested with the growth of tantalum pentoxide thin films. The model incorporates evaporation, diffusion, and convection phenomena in mass transport and a kinetic description of the surface reactions and film growth. The theory gives a quantitative estimate for the deposition rate and thickness uniformity of the film. The necessary conditions for an ALD operation are discussed. The experiments support the predictions provided a pure ALD mode is prevailing.