Base-pair and trimer formation between certain adenosine derivatives and anticancer agents has been studied by Fourier-transform infrared spectroscopy by determining hydrogen-bond association constants. The results indicate that the anticancer agents studied have a preference for Hoogsteen over Watson-Crick type base-pair formation and that they have an appreciable tendency for trimer formation. This might be the way in which they interfere with site recognition and hinder unwarranted protein formation.