Abstract: Polymorphism of active pharmaceutical ingredients (APIs) gets increasing attention as an important physico-chemical parameter influencing bioavailability and stability of API and pharmaceuticals. Co-crystals of API with common pharmaceutical excipients become very important as a tool to tune up solubility and absorption. Bisphosphonates (e.g. alendronate, risedronate, ibandronate) are widely used in clinical practice. They are indicated for the treatment and prevention of osteoporosis. They are powerful inhibitors of bone resorption, but their gastrointestinal adsorption is only about 1% due to their high hydrophilicity. Some experiments were designed to prepare co-crystals of risedronate. In the present study various mixtures of risedronate and excipients were prepared. All the prepared mixtures (solid compounds) and/or new entities were analyzed by means of FT-NIR, FT-Raman spectroscopy and solid state NMR.