The analytical characterization, by GC-MS, of individual compounds in mixtures of steroids, such as occur frequently in biological extracts, is difficult because of the close similarities in structure and properties of many components. The improved separating power of capillary (open-tubular) columns alleviates the problem, but does not solve it fully: for example, the coincidence of retention times of two different compounds may still be virtually complete. Comparative analyses on two distinctively different phases afford one valuable application of selectivity, but may not always be feasible when costly columns are required. Comparative analyses of the sample, before and after effecting its modification by well-defined reactions, are inexpensive and are particularly when selective transformations are used. The use of the microbial enzyme cholesterol oxidase as a selective oxidant for 3 beta-hydroxysteroids (chiefly limited to 4-ene, 5-ene and 5 alpha-types) is illustrated for a model mixture of androstanols related to the boar pheromone (5 alpha, 16-androsten-3 alpha-ol). Retention regularities and changes in mass spectra enhance the reliability of identifications. An exploratory application of cholesterol oxidase in the analysis of minor "polar" sterols in human serum is reported. Most of the known minor sterols are good substrates for the enzyme, and their transformation products yield distinctive GC-MS data, as exemplified for the 7 alpha- and 7 beta-hydroxycholesterols. Another convenient and versatile selective reagent is methaneboronic acid, which yields cyclic esters of suitably constituted diols. These derivatives have shorter retention times (on "non-polar" phases) than the di-TMS ethers, chiefly by virtue of their much lower molecular weights. The mass spectra of cyclic boronates generally show clear molecular ions, also fragmentations that complement the information obtainable from the di-TMS ethers. These features are illustrated for a group of diols and triols of the 5 alpha-pregnane series.