The absorption spectra have been examined of a large number of dyes of various chemical classes, including "reactive" dyes, in aqueous and non-aqueous solution, and adsorbed in solid transparent films. Almost all dyes have two absorption (x and y) wavebands. The shorter (y) waveband increases in height relative to the longer (x) with increase in concentration, in nearly all cases. This is attributed to aggregation. There is also a shift of both peaks with change in solvent ("solvatochromic" effect). The statistical significance of the method is examined.