The influence of composition, and added salts and polyols on the stability of an oil-in-water microemulsion formulation and on the apparent pK(A) (apK(A)) of the used oleate surfactant is investigated. High temperature favours a decrease of the apK(A) and leads to the formation of more hydrated micelles. The apK(A) decreases also when the percentage of ethanol increases. Citronellol molecules do not significantly influence the apK(A) at concentrations between 0% and 2% w/w. By contrast, with increasing limonene concentration, the apK(A) increases. It was observed that anions of sodium salts destabilize the microemulsion and high temperatures are needed to recover it. By increasing the concentration of NaCl, a slight increase of the apK(A) is observed, which can be associated with a non-specific, electrostatic (Debye-Huckel) effect. Cations of chloride salts have different effects depending on their ability to exchange with Na+ near the carboxylate group. Li+, Na+ and K+ have apparently a salting-out effect. Tetramethylammonium chloride and choline chloride have salting-in effects until respectively 0.6 and 0.4 mol kg(-1) The associations of sorbitol or glycerol with ethanol lead to a salting-in effect and to a decrease of the apK(A) of Na-Oleate. (C) 2013 Elsevier Inc. All rights reserved.