AbstractElectron impact mass spectral data for each of the four isomeric 16,17‐, 15,17‐ and 14,17‐diols of 3‐methoxy‐1,3,5(10)‐estratriene and the 15,17‐diols of 3‐methoxy‐14β‐1,3,5(10)‐estratriene are reported. The mass spectra of the diols show very similar fragmentation patterns except for differences in the relative abundances of particular ions. The different [M − H2O]+˙/[M]+˙ and [M − 2H2O]+˙ [M]+˙ ratios can be used for distinguishing between the four isomeric 3‐methoxy‐1,3,5(10)‐estratriene‐14,17‐diols as well as between the four isomeric 3‐methoxy‐14β‐1,3,5(10)‐estratriene‐15,17‐diols. No significant differences could be detected in the spectra of the epimeric 16,