A partially deuterated Pb-stearate multilayer prepared by means of the Langmuir-Blodgett technique on a Si support was investigated by X-ray and neutron reflectometry. It consists of a superstructure built by two protonated and two deuterated hydrocarbon chains. The X-ray reflectivity is sensitive to the gradient of the electron density between the hydrocarbon chains and the sheets of metallic counter-ions whereas the neutron reflectometry measures the gradient of scattering length density which is a maximum at the interface between the deuterated and protonated chains. Sharp Bragg peaks were found which are due to the periodic distance between the metallic sheets whereas the peaks responsible for the interface between deuterated and protonated chains are smeared out. The intensities of these peaks can only be explained assuming a partial exchange of deuterated and protonated chains between the sublayers.