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Different polymorphic forms of diflunisal are obtained by coevaporation with CS and CMCS.The kneading method gave rise to amorphous diflunisal-chitosan solid systems.The solubility of diflunisal increased markedly in the presence of CMCS.The highest increase of diflunisal release was obtained with CMCS.The lower CS molecular weight the higher diflunisal release.The interactions of diflunisal (DF) with chitosans (CS) of different molecular weights and carboxymethylchitosan (CMCS), a water-soluble derivative, have been investigated. The interactions in solution have been studied by solubility assays in which the highest solubilisation (13-fold) was obtained with CMCS. Solid dispersions were prepared by coevaporation and kneading methods. Solid state characterisation was performed by X-ray diffraction analysis, scanning electron microscopy, thermomicroscopy, differential thermal analysis and infrared spectroscopy. Drug–polymer electrostatic interactions and hydrogen bonds are the main binding forces in these systems. The kneading method gave rise to amorphous systems regardless of the polymer employed. However, coevaporation resulted in the formation of different polymorphs of diflunisal (form II or III) depending on the type of polymer used. Therefore, it seems that drug–polymer interactions determine the crystallization pattern of the drug. Finally, diflunisal release from these systems improved markedly with CMCS and significantly in the presence of low molecular weight CS.