(S)-N-(4-nitrophenyl)-prolinyl stearate (SNPP) and 2,4-dinitrophenyl-1-alanine stearyl ester (SMAP), stearyl derivatives of the molecules NPP and MAP (which form non-centrosymmetric crystals), have been prepared and studied as candidate materials for Langmuir-Blodgett films for non-linear optics. Pressure-area isotherms were determined under various conditions of sub-phase pH and temperature for both materials, separately and in mixtures with other film-forming species. Surface layers of the pure materials were at best metastable, and Langmuir-Blodgett films deposited from mixtures did not incorporate any SNPP or SMAP. Molecular modelling showed that in SNPP the hydrophobic tail appears to prevent stable film formation by preferring to lie over the weakly hydrophilic nitro group, while in SMAP the orientation of the tail is more favourable but the two nitro groups appear to destabilize the film.