Wet‐grown, baked 5400Aå films were controllably hydrated in tritiated‐water vapor under conditions approximating device operation (50°–250°C, 0.25–8 hr, 25–400 Torr water vapor pressure). Fast penetration and fast out‐diffusion suggest a micropore diffusion mechanism. This conclusion is supported by a low diffusion activation energy (0.3 eV) and exponential profiles in the bulk oxide. Strong surface peaking of the hydrogen tracer in both indiffusion and outdiffusion is attributed to surface energy lowering (Gibbs effect). “Water” under these conditions gave negligible MOS or even with a hydrogen concentration of at the silicon. Sodium contamination gave an order higher water entry rate. Water, likewise, increased sodium entry rate and charge production by the sodium.