Stimulated emission pumping-population transfer (SEP-PT) and hole-filling (SEP-HF) spectroscopies were used to determine the energy thresholds to isomerization between thirteen reactant -product conformer pairs in the biomolecule serotonin (SERO). Serotonin is a close structural analog of tryptamine (TRA), differing in having a hydroxyl group in the 5 position of the indole ring. A previous spectroscopic study (LeGreve; et al. J. Am. Chem. Soc. 2007, 129 (13), 4028) identified eight conformational isomers of SERO, whose interconversion involves motion of the 3-ethylamine side chain, the 5-OH group, or both. In the cases in which only an ethylamine side chain reorientation occurred, the barriers were found to be similar to, but systematically somewhat smaller than, those in TRA, which has been studied by similar methods (Dian; et al. Science 2004, 303 (5661), 1169; Clarkson; et al. J. Chem. Phys. 2005, 122 (2 1), Art. No. 214311). In most cases, the experimental thresholds are well reproduced by calculated transition states separating the conformational wells; however, tunneling effects may anificially reduce the thresholds observed for isomerization of SERO(A,Gpy(out)) and SERO(B,Gpy(up)) into SERO(C,Gph(out)). The A -> A' isomerization involving only the OH rotation from anti to syn was found to be 721-761 cm(-1), in accordance with the calculated classical barrier. For isomerizations in which the ethylamine side chain reorients as does the OH group, the barriers to isomerization were consistent with sequential rather than concerted motion of both groups. Finally, some evidence for mode-specific effects in the product quantum yields near threshold is presented.