The preparation of N-thiobenzoyl and N-phenylthioacetyl derivatives of L-valine, L-tyrosine, and, partly, L-leucine, as well as the corresponding salts, esters, and amides, is described. Ultraviolet spectra, as well as optical rotatory dispersion and circular dichroism curves are reported for the above compounds in water, methanol, diaxane, and isooctane for the wave length region above 275 m[mu]. In a number of cases unexpected and striking shifts in the sign of the observed Cotton effects are noticed on going from one solvent to another. The experimental results indicate that differently solvated species coexist in solution in a number of cases. The observed anomalies are discussed in terms of solvent-influenced changes in rotamer composition, possibly to the extent of the existence of solvent-dependent equilibria involving differently solvated species. Important conclusions of the present study are: considerable caution must be observed in assigning configuration to chiral molecules solely on basis of the sign of the Cotton effect in systems where the solute-solvent interaction is unknown, and even in a given solvent cases are recorded for homologous compounds, a identical chirality, exhibiting Cotton effects of opposite sign.