On-line LC-EC/ESI-MS has been established as a fast and simple method to mimic some types of oxidation reaction of various drugs and to study the formation and structure of the resulting products. This technique has been applied to a 2,6,9-trisubstituted purine, R-roscovitine, which is known to be an inhibitor of some cyclin-dependent kinases (CDKs) and a potential anticancer drug. Oxidation of R-roscovitine in an electrochemical cell (EC), operated under various conditions, resulted in appearance of 6 major products. These were further analyzed by high-resolution mass spectrometry, their structures were elucidated by accurate mass measurement and compared to previously identified R-roscovitine in vitro/in vivo metabolites. Although none of the observed products was structurally identical to those identified previously in vitro/in vivo, all of them, except for the methoxylated products, resembled similarity due to appearing through the same reaction type. R-roscovitine in the EC cell underwent N-dealkylation of the isopropyl moiety, hydroxylation of the aromatic side-chain, dihydroxylation, methoxylation and dimer formation. The hydroxylation product was identified as Olomoucine II, a R-roscovitine derivative, which displays 10-times higher CDK-inhibiting activity than R-roscovitine and the occurrence of which, as R-roscovitine product, has not yet been observed in vitro/in vivo.