Selenium vapour distribution and consumption during selenization process of Cu-In metallic layers provided in a quasi-closed system with inert gas environment at atmospheric pressure were studied. A model taking into account the four most important mechanisms of Se consumption, namely the creation and maintenance of Se vapour over-pressure, the out-diffusion through open boundaries of quasi-closed system, the drift transport by constant nitrogen flux and chemical reactions with Cu-In metallic layers, is developed to estimate the total amount of selenium required for selenization process. The diffusion mechanism was found to be dominating the selenium consumption for the used reactor design. Calculated values find a good agreement with experimental results from weighing of the Se source material before and after selenization.