Quantum yields for the photoisomerization of trans,trans-1,4-diphenyl-1,3-butadiene (tt-DPB), determined in benzene, cyclohexane, methylcyclohexane, hexane, and perfluorohexane, confirm the low values reported earlier for benzene and cyclohexane and reveal even lower values in the last two solvents. In contrast to trans-stilbene (t-St), fluorescence and torsional relaxation leading to photoisomerization do not account exclusively for S, tt-DPB decay. Competing radiationless singlet excited-state decay pathways exist in tt-DPB, which do not lead to photoisomerization and may not involve large-amplitude torsional motions. Our results invalidate analyses of tt-DPB fluorescence quantum yields and lifetimes that assign all radiationless decay to the isomerization channel. Gas-phase chromatography analysis of tt-DPB photoisomerization in hexane shows the reaction to be concentration-independent and reveals, for the first time, a significant, two-bond photoisomerization pathway, phi(tt -> tc) = 0.092 and phi(tt -> cc) = 0.020. The dominant one-bond-twist (OBT) process is accompanied by a bicycle pedal (BP) process that accounts for almost 20% of tt-DPB photoisomerization. The OBT tt-DPB photo isomerization quantum yield is largest in benzene (Bz) and smallest in perfluorohexane (PFH). Contrary to expectations, reduction in medium friction in PFH is accompanied by a decrease in phi(tt -> tc). The 1(1)B(u)/2(1)A(g) order and energy gap appear to control the contribution of torsional relaxation to radiationless decay. Lowering the 1(1)B(u) energy as in Bz favors photoisomerization. Reversal of the 1(1)B(u)/2(1)A(g) order in PFH is accompanied by short tau(f) and small phi(f) and phi(tt -> tc) values that suggest the presence of competing 2(1)A(g) -> 1(1)A(g) relaxation paths that are unproductive with respect to photoisomerization. We conclude that the Birks extension to diphenylpolyenes of the Orlandi-Siebrand cis-trans photoisomerization mechanism is not valid. Photoisomerization appears to occur in the 1(I)B(u) state, and we argue that this applies to t-St as well.