Neutron reflection is used to study the concentration profile at the interface between two thin layers of polymer. The first layer was prepared by spin casting on a silicon substrate; the second film was similarly prepared on a different substrate, floated off in water and placed over the first film. As prepared, the polymer-polymer interface has a contaminant film of water with approximately 0.26 mg m-2 surface coverage, which can be removed by annealing under vacuum at temperatures close to the glass transition temperature of the polymer. The annealed bilayered specimen does not show any trace of interdiffusion between the two polymers, even when the two polymers are chemically identical. © 1991.