Fluorenone sensitization in solution interconverts all-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) with its trans,cis,trans, cis,trans,trans, and cis,cis,trans (trace) isomers, tct-, ctt-, and cct-DPH, respectively. Photoisomerization quantum yields are reported for the three major isomers in degassed and air-saturated benzene. In degassed solutions the quantum yields are strongly concentration dependent due to quantum chain processes. The presence of air eliminates the quantum chain processes, as all DPH triplets are deactivated by oxygen. Triplet-triplet absorption spectra observed in the microsecond time scale starting from these three DPH isomers are identical. The concentration dependence of the DPH triplet lifetime is consistent with the concentration dependence of the isomerization quantum yields. The results indicate that in benzene DPH triplets exist as an equilibrium mixture of ttt, tct, and ctt isomers whose composition at 20 degrees C, 94% ttt, 5% tct, and 1% ctt is revealed by isomerization quantum yields :in the presence of air. Photoisomerization quantum yields in the presence of air show that the isomeric triplets are fully equilibrated within less than 100 ns.