Foods and biomass in general, need to be characterized chemically and physically in order to comply with regulations and to define processing conditions, quality, and stability of the products. Among the many analyses performed in foods, moisture content is typical and important. It plays a fundamental role in microbial stability, due to its effects on microorganims’ growth rate and survival. Other important factors affected by moisture content are food quality, including taste, appearance, and texture; and the general economics of food processing including storage, transport, and moisture conditioning for mixing and drying operations. Regulations usually put limits on the maximum and minimum water content, which are market and country specific. Clearly, understanding the moisture role on the properties of foods requires reliable measuring techniques, adequate methods for data analysis, and ideally, mathematical models with predictive capability. In this project, we use a molecular thermodynamics approach to equilibrium moisture content in complex substrates such as biomass, foods, and human tissue. The modeling framework includes water interactions besides adsorption such as mixing and swelling. Its parameters give physical insight on the nature of the water activity in foods, which is an important advantage compared to empirical fitting parameters.