SYNOPSISThough azo dyes are not fluorescent in solutions, many azo dyes become fluorescent when bound to tissues. This fluorescence is determined largely by the aromatic components of the dye molecules. Like chromophores, these fluorophores are affected by other groupings in dye molecules. Relatively short, compact systems of conjugated double bonds or linking of such resonators by insulating groups favours fluorescence. Long chains of conjugated double bonds and coupling of azo groups in the 1,4‐ or 2,7‐ position of naphthols diminish or abolish fluorescence. Various factors considered important for the fluorescence of other classes of dyes, such as ring closure, coplanarity of chromophores, accumulation of ring systems and low dye concentration are detrimental to the fluorescence of azo dyes. However, these rules apply only to azo dyes; the fluorescence of triphenylmethane, anthraquinone, and quinone‐imine dyes is governed by different factors. Three dioxazine dyes and three copper phthalocyanine dyes included in this study were non‐fluo