The mitochondrial membrane potential, mtMP, is the electric part of the protonmotive force, Δp_H+.

Δψ = Δp_H+ – Δµ_H+ / F

mtMP or Δψ is the potential difference across the mitochondrial (mt) inner membrane, expressed in the electrical unit volt [V].

mt-Membrane potential can either be measured in the NextGen-O2k Fluo and O2k-Fluo fluorometrically by using the fluorophores TMRM, Safranin, or Rhodamine 123, or potentiometrically with the O2k-TPP⁺ ISE-Module, which can be applied with any O2k configuration (Series B or higher), by using the ion reporter TPP⁺. All mentioned ion indicator molecules are cations which inhibit respiration, making it essential carry out respiratory controls to evaluate the effect of the indicator.