A physicochemical study of six crystalline modifications of the drug nitrofurantoin Is presented. These forms comprise two polymorphs, two monohydrates, a DMF solvate and a DMSO solvate. The clathrate nature of the solvates as well as their thermal decompositions are described. The results of dissolution rate measurements for the polymorphs and monohydrates are discussed in relation to the formulation of the drug.