Recently we reported a method to measure noise and impedance quasi simultaneously. The method was demonstrated in the determination of the symmetry factor of a single electron charge transfer process. In the present work we applied the noise + impedance measurement to a complex electrochemical reaction, namely to the hydrogen evolution process, which, in acidic media is supposed to take place via the Volmer-Heyrovsky mechanism. The theory of the noise corresponding to this mechanism has been derived by Tyagai in the early 1970s, however, for the verification no experiments have been published so far. Our investigation was based on the apparent electron number that we calculated from the cur-rent noise spectra with certain corrections. We found apparent electron number values between 1 and 2, depending on the electrode material and the applied external current. The results could be rationalized on the basis of Tyagai's theory. (c) 2007 Elsevier Ltd. All rights reserved.