The conversion of four alkyl nitrile compounds, RCN, to the corresponding carboxylic acids and ammonia in fluid H2O at 150-260 degrees C under 275 bar is described. The reaction rate was increased by the addition of 0.3-1.0 N HCl. The kinetics and pathways of the reactions were determined in real time by IR spectroscopy with a sapphire-titanium flow cell. Ex situ IR and Raman spectral data following batch reaction aided in establishing the reaction pathways. When R = CH3-, CH3CH2-, and (CH3)(2)CH-, the relatively simple conversion to RCO2H and NH3 occurred and was modeled by two protonation equilibria and two forward reactions. When R=HOC(O)CH2-, a more complex pH-dependent reaction took place because of the presence of two functional groups. The carboxylate group reacted at high pH, whereas the nitrile group reacted at low pH. The kinetics of these reactions were determined in real time, and the Arrhenius parameters were obtained. The activation energy for the hydrothermolysis of the nitriles qualitatively correlates with v(CN), indicating that the electron-donating power of R is a factor in the rate of reaction. Consistent with this observation is the fact that a Taft plot is linear when the electronic effect of R is weighed more heavily than the steric contribution. This was not the case when R=HOC(O)CH2-, where a different transition state is proposed.