18-Hydroxy-11-deoxycorticosterone forms a non-polar dimer under acidic conditions. The dimer was purified by high performance liquid chromatography and further analysed by electrospray mass spectrometry. The purified dimer has a molecular weight of 656 Da with two free ketone groups and no free hydroxyl functions. The mass spectrometric data suggest a 20,21,20',21'-anhydro dimer structure for the dimer. Microbore high performance liquid chromatography/electrospray mass spectrometry of rat adrenal extracts indicated that both 18-OH-B and 18-OH-DOC were present, although no 18-OH-DOC dimer was detected.