In arid environments, porous media are unsaturated with water which is submitted to capillary constraints. The present chapter focuses on the geochemical impacts of such physical constraints and how theoretical analysis can help interpreting field or laboratory observations. The basic principles of capillary geochemistry suggest that the fate of contaminants, either organic or inorganic, can be significantly impacted in terms of reactive mass transfer in addition to flow and transport processes. All these mechanisms are closely interconnected, what makes the description of the behavior of such systems very complicated. An important work still has to be done in order to achieve such a goal: a number of mechanisms are not taken into account in the current state of development of the capillary geochemistry, namely mechanisms that occur in the thinnest confining geometries, like disjoining pressures, surface forces, etc.