The depth distribution of water below the surface of some Mesolithic flint pieces has been studied by means of a nuclear reaction between a ₁₅N beam and the hydrogen of the water. Freshly exposed surfaces show a bulk molecular concentration of approximately 1.3% water, while the Mesolithic surface contains three times as much water within the first 800 Ã…. The excess concentration trails slowly off to reach the bulk concentration more than 1 mum below the surface. Pieces of flint have been soaked in water for 90 hours at different temperatures. The relative weight increase is between 0. 1% and 0.2%, independent of size, indicating an increased volume concentration of approximately 0.5 molecular percent. This concentration is confirmed by nuclear-reaction profiling. Again, the soaked flint pieces show an excess water concentration, which, surprisingly enough, was also found at surfaces exposed immediately after soaking. It is inferred that the flaking of the flint occurs preferentially along internal water-filled cavities, which partly explains the different workabilities of dry and wet flint. See also 20-32. -- AATA