The effects of confined geometry and substrate interaction on the short time interdiffusion in polymer films have been investigated by neutron reflectometry. Samples consist of alternating layers of protonated and deuterated polystyrene. The interface broadening of a thick film (4.4 R g thick) confined between two thick films, a thinner film (0.78 R g thick) confined between two thick ones and a thin film (0.77 R g thick) deposited directly on a glass substrate with a thick film on top is measured as a function of time. R g is the bulk radius of gyration, which is taken as a measure of the relative film thickness compared to the chain dimension. Samples are subsequently annealed at temperatures slightly above the glass transition temperature, T g, and the interface widths as a function of time are determined from model fits to neutron reflectivity curves. Neither a relaxation process at small times nor a large change of segmental dynamics as a function of film thickness are observed. Only a slowing down of interdiffusion dynamics for the thin film between two thicker ones is detected as well as a still smaller interdiffusion for the thin film placed directly on the substrate, which is attributed to polymer–substrate interaction.