The effect of polymers [gelatin, dextran, poly(viny1 alcohol), polyvinylimidazoles] on the photochemical and thermal isomerization of the water-soluble bisazo dye, Acid Yellow 38, has been studied. The absorption spectrum curve of the cis AY38 isomer was obtained using the method of Fischer. We have shown that the rate of thermal cis-trans isomerization of AY38 is first order and independent of pH from 6 to 9. An activation energy of 84 & 4 k J mol-l has been calculated for this process. The effect of added polymers on the rate of this conversion is relatively small. The quantum yields for the trans-cis and cis-trans conversions have been estimated to be 0.15 0.01 and 0.34 F 0.03, respectively, for irradiation at 380 nm. Only polyvinylimidazoles were found to influence the behavior of the dye to any degree. For quaternized polyvinylimidazole the polymer/dye ratio determined solution turbidity, wavelength of maximum absorption, molar absorptivity, and the degree of conversion at photoequilibrium. The effect of polymer was interpreted in terms of increased dye/dye interactions due to high local concentration of the dye bound to polymer. The charge neutralization and crosslinking by the dye due to ionic interactions are seen to be the cause of polymer/dye complex precip itation. Precipitation occurs over a narrow [polymer]/[dye] ratio range.