Spectroscopic and gas-chromatographic observations were done on the chromogenic reactions of testosterone (I) with Brφnsted acids such as sulphuric acid, phosphoric acid, and perchloric acid and with Lewis acids such as SbCl3, SbCl5, ZnCl2, and AlCl3. Although the spectra due to the reactions with various Brφnsted acids were different in each of the colorimetric procedures for I, the residues obtained from the reaction mixtures gave a similar spectrum with each other, when they were submitted again to the colour development at room temperature in the dichloromethane solution saturated with 70% HClO4 (Fig. 1). From this and the results of gas chromatography (Fig. 2) of these residues, 17-methyl-18-norandrosta-4, 13 (17)-dien-3-one (II) and/or 17-methyl-18-norandrosta-4, -13 (14)-dien-3-one (III) were/was likely to be the intermediary chromogen (s) commonly present in these colorimetric reaction mixtures. Lewis acids in the reactions with I, on the contrary, gave entirely different results as shown in Fig. 2 and 3. In contrast to I, the dienones, II and III, showed readily at room temperature the bathochromic absorption with HClO4 as well as SbCl5 in dichloromethane, which was markedly intensified in the presence of oxidizing agent (Fig. 4).