Neutron reflectivity has been used to determine the thickness and surface coverage of monolayers of two 14-residue beta-hairpin peptides adsorbed at the air/water interface. The peptides differed only in that one was labeled with a fluorophore, while the other was not. The neutron reflection measurements were mainly made in null reflecting water, NRW, containing 8.1% D2O. Under this isotopic contrast the water is invisible to neutrons and the specular signal was then only from the peptide layer. At the highest concentration of ca. 4 mug/mL studied, the area per peptide molecule (A) was found to be 230 +/- 10 and 210 +/- 10Angstrom(2) for the peptides with and without a BODIPY-based fluorophore, respectively. The thickness of the peptide layers was about 10 Angstrom for a Gaussian distribution. With decreasing bulk peptide concentration, both surface excess and layer thickness showed a steady trend of decrease. While the neutron results clearly indicate structural changes within the peptide monolayers with increasing bulk concentration, the outstanding structural feature is the formation of rather uniform peptide layers, consistent with the structural characteristics typical of beta-strand peptide conformations. These structural features are well supported by the parallel measurements of the adsorbed layers in D2O. With this isotopic contrast the neutron reflectivity provides an estimate about the extent of immersion of the peptide layers into water. The results strongly suggest that the 14-mer peptide monolayers were fully afloat on the surface of water, with only the carboxy groups on Glu residues hydrated.