Methanogenic ecosystems function close to oxidation– reduction equilibrium with change in Gibbs free energy (ΔG) values in the range of −10 to −20 kJ mol−1 (Hoehler et al., 1998; Jackson and McInerney, 2002). With such small margins, attention to detail when making free energy calculations becomes exceedingly important. A pervasive source of confusion, if not error, when making free energy calculations is the fact that organic acids and their anions occur simultaneously (Mazur et al., 2003; Mun et al., 2008). Here, we point out that such confusion is not necessary, as weak acids and their conjugated bases are in thermodynamic equilibrium in anaerobic ecosystems. This equilibrium implies that Gibbs free energy values for redox reactions are to be calculated by Gf values of either the acid, with the formula Gf=Gfo+RT lnα, or the conjugated base with the formula Gf=Gfo+RT ln(1−α), where α=10−pH/(10−pH+10−pK); by incorporating either the acid or its conjugated base the counterpart is incorporated implicitly.