In this paper we present an experimental investigation in the drying of clay material. Three type of purified clay are used in this study. We work under different experimental conditions changing the temperature or the relative humidity of the drying air. Drying kinetics and diffusion coefficient are presented and commented on. Generally the experimental determination of the diffusion coefficient as a function of moisture content is difficult. Very often drying curves are used; however, a functional relation between diffusion coefficient and moisture content has to be assumed. This functional relation is unknown, since finding such a relation was the purpose of the experiment in the first place. An analysis of diffusion coefficients determined from drying curves shows that no unambiguous results can be obtained; i.e. the diffusion coefficient depends on experimental conditions.