Neutron reflectivity (NR) was applied to measure the concentration of water at the buried interfaces between an amorphous polyimide and silicon single crystal wafers. Excess water was discovered within 30 Angstrom of the metal/polymer interface, where the water concentration reached 17% (by volume) for the samples without a coupling agent and 12% for the ones with coupling agent. Beyond the interface, the water concentration was measured at 2 to 3%, which is typical of bulk polyimide. The above results demonstrate conclusively the unique power of NR in determining water concentration near a buried interface, and provide the first quantitative evidence for a water concentration profile which peaks in the interface region.