The hydrogen evolution reaction of HCOOH, CH3COOH, and C2H5COOH solutions consists of two different reduction processes depending on the evaluated potential region; (1) independent reduction of RCOOH and (2) simultaneous reduction of RCOOH and H2O. The reduction of each carboxylic acid generates an apparent convective diffusion-controlled limiting current. The first achievement of the present study is that by using a rotating disk electrode (RDE) and a sinusoidal hydrodynamic modulated-rotating disk electrode (SHM), it was elucidated that the additive property of the reduction currents of RCOOH and H2O was not effective, and the convective-diffusion current was successfully distinguished from the total current. The second achievement is the successful analysis of the rotation-speed dependency of the limiting current in RDE and SHM using a modified theory of the Koutecky-Levich equation. The slopes of the plots for each carboxylic acid increased in the following sequence: RDE, SHM (p = 0.05), and SHM (p = 0.24), which is consistent with the theory. The dissociation rates of the carboxylic acids and the reverse recombination rate were calculated. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.