The replacement of hydrogen with deuterium is frequently used in conjunction with neutron diffraction to investigate peptide-membrane interaction. This isotopic substitution in an amino acid residue radically changes the neutron scatter pattern of the peptide, thereby allowing its localisation within the bilayer with the aid of derived Fourier maps. Nonetheless, this technique relies on the generally held assumption that normal and isotopically enriched protein species do not differ significantly in structure or biological activity. Recently, this assumption has been questioned and here, diffraction data from studies on a membrane interactive peptide clearly challenge the reliability of this assumption.