A method was developed to study the correspondence of color change with structural change of the polyenyl cation in the color reaction of cholesterol in strong acid. Five unsaturated steroids having conjugated systems consisting of 3, 5-diene (I), 3, 5, 7-triene (II), 4, 6, 8(14)-triene (III), 3α, 5-cyclo-6, 8 (14)-diene (IV) and 2, 4, 6-triene (V), and four bisteroids having 3, 3'-linkages and conjugated systems consisting of 3, 3', 5-triene (VI), 3, 3', 5, 5'-tetraene (VII), 3, 3', 5, 5', 7-pentaene (VIII) and 3, 3', 5, 5', 7, 7'-hexaene (IX) were prepared and their electronic spectra were measured in trifluoroacetic acid (TFA), in a CHCl₃ solution of TFA, in H₂SO₄ and in 80% H₂SO₄. The absorption maxima observed immediately after the preparation of the solution and those observed after standing for various times at room temperature were correlated with the chemical species presumed to be produced in strong acids. The following chemical conversions were found to take place : ready conversion of I to VI and VII, and of VI to VII ; very rapid conversions of both II and IV to III ; ready conversion of V to a dimeric molecule ; conversion of III to a dimeric molecule. The color change of cholesterol in strong acid was found to be mainly due to the formation of VI and VII and to their chemical conversions.