AbstractThe concentration dependence of the activation volume ΔV≠ and the free activation enthalpy ΔG≠ for the inversion of the dimethylamino‐group has been determined in a series of dimethylamides in aqueous solution by high pressure HRNMR (pmax = 150 M Pa). The compounds studied are the N, N‐dimethylamides of formic‐, acetic‐, propionic‐, isobutyric‐, valerianic‐ and benzoic acid. In all nonaqueous solvents ΔV≠ is only slightly concentration dependent, ΔV≠ being in the range of +10 ± 2 cm3·mol−1. For all compounds studied, ΔV≠ in the dilute aqueous solutions (x ≥ 0.9) decreases very rapidly to values around + 2 cm3·mol−1. It is attempted to explain this decrease by the formation of an open hydration shell around the dimethylaminogroup and to correlate the concentration effects with the hydro‐phobic character of the amides.