The absorption and fluorescence maxima of 20 isoflavones have been determined in methanol solution and the effect of addition of water, 50% sulfuric acid, aluminium trichloride, borax, sodium acetate, ammonia and sodium hydroxide has been studied. The following findings may be useful in the structure elucidation of naturally occurring isoflavones: (a) 5-Hydroxyisoflavones have band I absorption maxima around 335 nm. 6-hydroxyisoflavones between 310 and 330 nm, and others below 310 nm. (b) Addition of water produces practically no shift in the absorption spectra, but - unlike other hydroxyisoflavones - can give distinctly longwave shifted new fluorescence bands with 7-hydroxyisoflavones. (c) Addition of sodium acetate gives rise to anion absorption of 7-hydroxyisoflavones and to partial anion absorption of 6-hydroxyisoflavones; the spectral maxima of 5-hydroxyisoflavones remain practically unchanged, (d) Ammonia gives rise to anion absorption of both 6- and 7-hydroxyisoflavones, but not of the 5-hydroxy isomers, (e) Sodium borate is a useful reagent to identify 6,7-dihydroxyisoflavones by virtue of its ability to form a chelate complex with an absorption maximum that is different from the anion absorption, (f) Aluminium trichloride forms complexes with both 5-hydroxy- and 6,7-dihydroxyisoflavones with distinct absorption maxima, (g) 5,7-D ihydroxyisoflavones may be recognized by addition of ammonia, which does not result in a longwave shift, but rather in an intensification of the longwave absorption band, (h) 6-H ydroxyisoflavones can be differentiated from the 7-hydroxy isomers by their longwave shifts (4 0-60 nm) following addition of ammonia. The respective shifts of the 7-hydroxy isomers are smaller, (i) 5-H ydroxyisoflavones are practically non-fluorescent. while others have fairly strong fluorescences, (j) The absorption and fluorescence maxima of isoflavones give unique combinations which may be useful in their identification, (k) Addition of aluminium chloride makes the non-fluorescent 5-hydroxyisoflavones fluorescent. (1) As in the case of absorption, 6,7-dihydroxyisoflavones form complexes with borate possessing fluorescence bands with maxima different from those of the anion bands