The mass diffusion coefficient is obtained using real-time holographic interferometry. The stresses induced during desorption and absorption of moisture are monitored as a function of time at various relative humidities. A mathematical relationship is applied to relate these stresses to the moisture diffusion through the thickness of the films. The advantage of the holographic interferometry technique is that both the swelling stress as a function of time and the mass diffusion coefficient can be quantified simultaneously during the same experiment. Several films and coatings were investigated : polyimides (PMDA-ODA and BPDA-PAA), photographic gelatin, and cellulose acetate. Each exhibited characteristic Fickian diffusion behavior. A gravimetric technique was employed to validate the holographic interferometry results. The moisture diffusion coefficients determined by holographic interferometry agreed well with reported Literature values.