The derivation and discussion of these equations originates in Erdey-Grúz 1970 and Bockris and Reddy 1972 . These relationships have been guiding electrochemists since then and were substantiated by Guidelli et. al. in a recent IUPAC technical report . Unfortunately, these equations are incorrect due to the inappropriate application of the quasi-equilibrium (QE) assumption instead of the steady state condition (Eq. 9a). The QE assumption dictates that the oxidation and reduction currents of any non-rds step are equal. While both currents are large compared to their difference, the steady state condition dictates that this difference is equal to that of the overall current divided by its respective occurrence number and consequently cannot be neglected. Simply put, using the QE assumption leads to an incorrect rounding operation by treating Eq. 10c as an equality rather than an approximation. In Fig. we display one of the original derivations for the estimation of Tafel slopes using the QE assumption side by side to an analogous derivation using the steady state condition. When employing the steady state condition, no information concerning the number of preceding steps remains in the exponent, invalidating the further derivations that eventually led to Eq. 18b-c using the QE assumption. Differences arising from the QE and the steady-state condition have been noticed previously by Marshall , but without highlighting their significance for the estimation and interpretation of kinetic parameters. For illustration, we demonstrate this error with a hypothetical example: We consider a reaction with two one-electron-transfer steps where the first step produces two intermediates and in the second step each intermediate is oxidized to the final product.