Gemfibrozil (GEM) is a xylyloxi-valeric acid derivative with an antilipaemic effect. This molecule has very poor water solubility, and the ability of different cyclodextrins (CDs) to form inclusion complexes was therefore tested. Primarily heptakis-2,6-di-O-methyl--cyclodextrin (DIMEB) was found to increase the solubility. The preparation of the complex in solid form for characterization was successful by means of physical mixing, kneading, spray-drying and ultrasonic treatment. The dissolution and in vitro membrane diffusion of the products were investigated. The n-octanol/water partition coefficients were determined for pure GEM and the DIMEB inclusion product, and the interactions leading to complex formation between the components of the products were examined by thermoanalytical methods and FTIR spectrophotometric analysis.