The circulating levels of 16β hydroxydehydroepiandrosterone (16β OH-DHEA) are at the limit of detection (less than 10 pg/mL), unlike the serum concentrations of 16α-hydroxydehydroepiandrosterone (16α OH-DHEA, 10–300 pg/mL) in premenopausal, postmenopausal and male serum. A major reason could be the rapid conversion of 16β OH-DHEA to 5-androstene-3beta, 17beta-diol 16 one (3β, 17β-diol 16-oxo) in serum due to the stereospecific structure of 16β OH-DHEA. In ultrapure H2O, there is no apparent conversion observed while 16β OH-DHEA (10 ng/mL) spiked in stripped or unstripped serum is quickly converted to 3β, 17β-diol 16-oxo at room temperature. During this conversion, a further converted product was observed with a difference in molecular weight of 16 Da from that of 16β OH-DHEA and 3β, 17β-diol 16-oxo, which could be their hydroxylation product, i.e. triol-ketone. Under basic conditions, further conversion occurs. The present data can explain the practically undetectable concentration of serum 16β OH-DHEA while 3β, 17β-diol 16-oxo is at the level of less than 50 pg/mL. Serum concentrations of (0.0–9.9 pg/mL for 16β OH-DHEA, 8.9–50.7 pg/mL for 3β, 17β-diol 16-oxo and 10.0–285.0 pg/mL for 16α OH-DHEA are measured in sera of premenopausal, postmenopausal women and men over 50 years of age.