A nonmonotonic, decay-growth-decay type forced Rayleigh scattering (FRS) decay profile was observed in the study of methyl red to-MR) diffusion in polymer solutions in which the carboxylic hydrogen of o-MR can form a hydrogen bond with polymer chain segments (Lee and Lodge, J. Phys. Chern. 1987, 91, 5546; Lee et al. Macromolecules 1992, 25, 6977). The diffusivities of ground state o-MR (trans form) and of its photoexcited isomer (cis form) are different enough to show the nonmonotoic decay profile, but the origin of the behavior has not been fully understood. In order to elucidate this observation, the dependence of the decay profile shape on polymer concentration as well as on tempreature was investigated by FRS. From the results, we propose that the internal hydrogen-bonding strength between the carboxylic hydrogen and the azo nitrogen in o-MR changes upon photoisomerization from the trans to cis form, which results in the different diffusivities of the two isomers in hydrogen-bonding media. This proposition is supported by ab initio calculations on the molecular structure of o-MR.