The in-situ conformations of peptide layers formed from the adsorption of two different synthetic 15-mer peptides at the hydrophilic silicon oxide/aqueous solution interface have been determined using neutron reflectivity (NR). The first peptide is based on the native sequence of a protein-binding domain within a heteromeric transcriptional activator, HAP2, identified from yeast Saccharomyces cerevisiae, with tyrosine (Y) present at the 1 st, 8th and 15th amino acid positions, hence we denote this YYY15. Substitution of tryptophan (W) at the same locations gives WWW15. Both peptides have a-helical structure in phosphate buffer, as determined by circular dichroism (CD) spectra. D2O was used as solvent in the NR experiments to highlight structural heterogeneity across the hydrogenated peptide layers. At pH 7, YYY15 was found to form a weakly adsorbed interfacial monolayer. However, the mutant WWW15 showed strong interfacial adsorption, with the interfacial layer characterized by a middle hydrophobic sublayer of 7-8 Angstrom with lower scattering length density and two almost symmetrical hydrophilic outer sublayers of 6-8 Angstrom with higher scattering length density, suggesting the formation of a "sideways-on" helical conformation. An increase in pH to 9 resulted in the improved packing within the interfacial layer with similar structure. However, decrease in pH to 5 reduced the interfacial adsorption, mainly due to the enhanced solubility of the peptides associated with the protonation of arginine (R) and lysine (K) groups and the decreasing concentration of divalent HPO42- in the phosphate buffer. Subsequent assessment of the reversibility of adsorption showed that once the peptide layers were formed they did not desorb. These interfacial structures may provide feasible routes to interfacial nano-templating.