The influence of alkyl chain length on the interfacial width of the n-alkane-water interface was examined with molecular dynamics simulations. We examined systems of n-hexane-water and n-nonane-water and also investigated the effect of different systems sizes. The electron density profiles of the systems were extracted from our simulations, along with corrections made for capillary waves, to estimate the X-ray reflectivity and the interfacial width. It was consistently found that increasing the alkyl chain length decreased the interfacial width in contrast to X-ray reflectivity experiments. Some of the potential reasons for this discrepancy are examined and discussed. (C) 2012 Published by Elsevier B.V.