We show that, by the influence on the gauche content in the acyl chains, the effect of very low concentrations (order 10-2 %) of penetrants (here 2,4- dichlorophenol) in lipid bilayers (here dipalmitoyl- glycerophosphatidylcholine) can be experimentally seen in the Raman spectra. The observed effects suggest increased order and are in accordance with previous X-ray diffraction and calorimetric data. These findings show great promise for studying in situ the effect of low amounts of various small molecules on cell membranes using Raman spectroscopy.