Disclosed is a process for the preparation of a bis(perfluoroalkyl-alkylthio)alkanoic acid of the formula ##STR1## wherein each R.sub.f is independently perfluoroalkyl of 3 to 18 carbon atoms or perfluoroalkoxyperfluoroalkyl of 3 to 18 carbon atoms, or mixtures thereof;each A is independently alkylene of up to 6 carbon atoms or said alkylene interrupted by --O--; N(H)-- or --N(lower alkyl)--;R is hydrogen or alkyl of 1 to 6 carbon atoms;R.sub.1 is hydrogen or lower alkyl; andR.sub.2 is hydrogen, lower alkyl, aryl or aryl substituted lower alkyl;by reacting a perfluoroalkyl-alkyl mercaptan of the formulaR.sub.f --A--SH (II)with a lactone of the formula ##STR2## in the presence of a catalytic amount of a Lewis acid to form a perfluoroalkyl-alkylthio lactone of the formula ##STR3## and reacting said perfluoroalkyl-alkylthio lactone with additional perfluoroalkyl-alkylmercaptan of the formulaR.sub.f --A--SH (IIa)in the presence of a catalytic amount of a Lewis acid, to obtain said bis(perfluoroalkyl-alkylthio)alkanoic acid of formula (I).