It is shown that many dehydration rate constants of carbonyl compounds (acetaldehyde, pyruvic and glyoxylic acids, formaldehyde) found by various kinetic approaches to do polarography are overestimated. This anomaly is connected neither with the adsorption effect, nor with neglecting the spherical diffusion. It is caused by the influence of the diffusion contribution to the total limiting kinetic current of the depolarizer from the solution bulk. Quantitative consideration of this effect has allowed us to obtain the correct rate constants under steady-state conditions, independently of the degree of the diffusion contribution.