From the reddish reaction mixture of 3, 26-dimethoxy-furost-5, 20-diene (ψ-diosgenin dimethyl ether, 2) with Ehrlich reagent, a colorless condensation product was isolated and characterized as 3, 26-dimethoxy-23-(p-dimethylaminobenzylidenyl)-furost-5, 20-diene (3). However, a coexisting red product could not be isolated because of its instability. On treatment with dried hydrogen chloride or conc. hydrochloric acid, the colorless chloroform solution of 3 readily changed into a reddish solution, which was reversibly decolorized on addition of 3% ammonia-chloroform solution. Based on ultraviolet and carbon-13 nuclear magnetic resonance spectral analysis, the coloration mechanism of 2 with Ehrlich reagent has been inferred to be as follows. The reaction is initiated by condensation between p-dimethylaminobenzaldehyde and 2 to form 3, followed by protonation to form an iminium cation (3B) under acidic conditions.