A N-14 and H-1 NMR spectroscopic study was carried out to shed light on microscopic aspects of the reaction of model alkylamines at a supercritical temperature of 400 degrees C. It is disclosed that NH3 and ROH (R = CH3CH2 and CH3 (CH2)(3)) are initially produced from the hydrolysis of ethylamine and butylamine, respectively. When the water density is doubled from 0.2 g cm(-3), the pseudo-first-order reaction rate is markedly enhanced beyond the linear response. It suggests that the transition state of the C-N bond cleavage is in a dipolar (ionic) state that can be more stabilized due to the many-body solvation by highly polar water molecules at a higher density.