Optical activity in second harmonic reflection from the air/water interface of solutions of simple tryptophan (trp) derivatives has been investigated. The derivatives include D- and L-optical isomers of Boc-trp and DD-and LL-isomers of the dipeptide Boc-trp-trp, where Boc is tert-butyloxycarbonyl. An experimental method based on polarization modulation of fundamental radiation by rotation of a quarter-wave retardation plate has been used to completely characterize the second-order nonlinear susceptibilities of the chiral surfaces, for the fundamental wavelength lambda = 564 nm. Boc-trp and Boc-trp-trp show similar chiral contributions to the nonlinear susceptibilities, which indicates that the chiral response does not depend on intramolecular interactions between trp residues. The efficiency of second harmonic reflection has been shown to be resonantly enhanced at the two-photon level, in the region of the near-UV bands (similar to280 nm) of the indole chromophore of tryptophan. The origin of the chiral response is discussed in terms of simple microscopic models, and a comparison is made with optical activity of tryptophanyl compounds in linear spectroscopy. The peptide gramicidin, incorporating several trp residues, showed no evidence of optical activity in second harmonic reflection from the air/water interface. This indicates that second harmonic optical activity is sensitive to structural effects such as conformational and orientational distributions of trp residues at the interface.