The assessment of weak acid membrane permeability (Pm) frequently involves large unilamellar vesicles. It relies on measurements of the intravesicular pH drop, pHin, in response to a sudden augmentation of external acid concentration. However, pHin may be primarily governed by non-instantaneous protonation and deprotonation reactions of (i) the acid itself, (ii) the buffer molecules, and (iii) the fluorescent pH reporter dye. Moreover, buffer concentration and acid gradient also serve as determinants of pHin, as we show here. The uniexponential time constant () of pHin(t) is an invalid measure of Pm as Arrhenius plots of Pm and reveal different activation energies for acid influx. We calculate Pm by fitting a mathematical model to experimental stopped-flow traces. The model takes into account not only the time course of total internal buffer capacity but also (i) water self-dissociation, (ii) volume changes due to acid induced osmotic water flow, and (iii) the spontaneous membrane proton leak. It allows extracting a Pm of 30.8 3.5 m/s for formic acid for 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles.