Because psychrometry takes into account a great number of variables reflecting the quality of the drying air, it is an interesting tool to improve the control and the optimization of the spray drying process. In this article the authors study the evolution of the psychrometric variables according to the values taken by four inlet parameters (inlet air temperature, liquid flow rate, solid concentration of the spray dried liquid, and nature of the product). The results highlighted the existence of mathematical models making it possible to optimize the process, but also to underline the influence of the nature of the product on the drying mechanism.