The ATP synthase is the key player of Mitchell’s chemiosmotic theory, converting the energy of transmembrane proton flow into the high-energy bond between ADP and phosphate. The proton motive force that drives this reaction consists of two components, the pH difference (∆pH) across the membrane and transmembrane electrical potential (∆ψ). The two are considered thermodynamically equivalent, but kinetic equivalence in the actual ATP synthesis is not warranted and previous experimental results vary. Here we show that, with the thermophilic Bacillus PS3 ATP synthase that lacks an inhibitory domain of the ε subunit, ∆pH imposed by acid-base transition and ∆ψ produced by valinomycin-mediated K+ diffusion potential contribute equally to the rate of ATP synthesis, within the experimental range examined (∆pH –0.3 to 2.2, ∆ψ –30 to 140 mV, pH around the catalytic domain 8.0). Either ∆pH or ∆ψ alone can drive synthesis, even when the other slightly opposes. ∆ψ was estimated from the Nernst equation, which appeared valid down to 1 mM K⁺ inside the proteoliposomes, thanks to careful removal of K⁺ from the lipid.