Abstract: The absorption spectra of 16, cyanine dyes in 18 solvents of various refractive index and polarity have been investigated in this work. It was found that the positive sol-vatochromism of the M-band of the molecules was not only dependent upon the refractive index n of the solvent used, but also on the dielectric constant ε. In correlating the band shift (in cm^-1) with the function f(n,ε) = (n²-1/2n²+1) + α(ε-1/ε+2) better linearity has been obtained. The parameter a in this function has a value less than unity, its sign was determined by the sign of solvatochromism of the system studied. In water containing bi-liquid system, the negative solvatochromism of the dimeri-zed dye molecules was also in good linearity with solvent molecules, the dimerized dye molecules can be treated as an isolated optical element.