MOLECULAR films with predetermined layered structures can be engineered by using techniques such as the Langmuir-Blodgett method(1,2) and self-assembly(1,3-9) to deposit discrete monolayers sequentially on a substrate. Such films might have a variety of uses-as smart surface coatings, nonlinear optical materials and in tribology, for example. Were we report the replicative grow th of a molecular film of self-assembling silane bilayers with hydrogen-interlayer polar regions into which further identical bilayers can be intercalated. The intercalation step is triggered by a chemical treatment and so can be carried out controllably, allowing duplication in one step of an entire multilayer structure. In this way, we can achieve the stepwise exponential growth of a multilayer film with a predictable number of stacked bilayers.