Despite the importance of gas/liquid/solid multiphase systems in the production of chemicals on an industrial scale, measuring the concentration of gases dissolved in the liquid phase-a prerequisite for determining basic reaction data such as rate and adsorption constants-remains challenging. Recently, a new permeation probe became available, which allows in situ measurement of gas concentrations in liquids. To evaluate potential applications, the probe was used to follow the concentration of dissolved hydrogen during the cobalt-catalyzed reduction of an aliphatic dinitrile to the corresponding diamine. The changes in the hydrogen saturation level during the reaction were compared to the gas-liquid (G-L) mass transfer characteristics of the reactor as determined by k(L)a measurements. Under the reaction conditions used, G-L mass transfer became the rate-determining step when the stirring speed was decreased. The permeation probe allowed for evaluating the significance of G-L mass transfer in a straightforward manner.