Thermodynamics of complexation of 5-nitroimidazoles with beta-cyclodextrin and its methylated and hydroxypropyl derivatives in water and in 0.25% polyvinylpyrrolidone are determined by solution calorimetry. A 1:1 stoichiometry was established. The equilibrium constant (K) for all the nitroimidazoles fall in the range 1000-1900 m(-1) suitable for use of cyclodextrins as drug carriers. The complexation ability is significantly enhanced by methylation of the beta-cyclodextrin. The stability constant increased in the order metronidazole < ornidazole < timidazole < secnidazole. The presence of polyvinylpyrrolidone enhances the stability constants. (c) 2007 Elsevier B.V. All rights reserved.