The aim of the present study was to characterize some compounds resulting from storing codeine aqueous solutions under different conditions in search of an impurity observed in some codeine preparations and in the urine of their users. Acidic, neutral and basic aqueous solutions of codeine were stored in the dark or exposed to sunlight for one month. Aliquots of the solutions were withdrawn at 3-day intervals, extracted with organic solvent at basic pH and analyzed by GC-MS before and after trimethylsilylderivatization; where applicable separated compounds were identified by database mass spectral libraries. Other compounds were tentatively characterized from GC-MS data and reaction mechanistic interpretations. At neutral pH, codeine was partially oxidized to norcodeine and codeinone while at basic pH only the former compound was formed. In both cases, light had no effect. In the acidic solutions exposed to light, codeine was autoxidized to two products which were tentatively characterized as epimeric forms of 10-hydroxycodeine. None of the three products was detected in pharmaceutical preparations of codeine or in opium. The nature of the oxidation products of codeine in aqueous solution is determined by the state of protonation of the molecule as well as by the presence or absence of photo effects. The results indicated that codeine aqueous pharmaceutical preparations should always be protected from light. On the other hand the stability of codeine in opium is most probably due to the co-presence of compounds with antioxidant properties.