1,2-Dithiolane-3-pentanoic acid (D,L-thioctic acid) of the formula ##STR1## is prepared by a process comprising (a) reacting a 2-(3-alkylthiopropionyl)-cyclopentanone-1 of the formula ##STR2## where R is a C.sub.1 -C.sub.4 alkyl, phenyl or benzyl in aqueous alkaline solution at a temperature of about 20.degree. C. to about 90.degree. C. to form the corresponding carboxylic acid of formula VI ##STR3## (b) reacting the compound of formula VI with an alkyl mercaptan at a temperature between -20.degree. C. and 0.degree. C. to form the corresponding thioketal of formula VII ##STR4## (c) reacting the compound of formula VII with sodium in liquid ammonia at temperature temperture between -60.degree. C. and -10.degree. C. to form the 6,8-dimercaptooctanic acid of formual VIII ##STR5## (d) reacting the 6,8-dimercaptooctanoic acid of formula VIII in alkaline solution with an iron (III) salt and oxygen to form the 1,2-dithiolane-3-pentanoic acid of formula IX, or in place of steps (a) through (c) reacting an acid of formula XII ##STR6## where R.sub.1 and R.sub.2 are hydrogen, C.sub.1 -C.sub.4 -alkyl, phenyl or benzyl, with the proviso that both R.sub.1 and R.sub.2 cannot be benzyl, with sodium in liquid ammonia at a temperature between -60.degree. C. and -10.degree. C. to form the corresponding 6,8-dimercaptooctanoic acid of formula VIII. The compounds of formulae VI, VII, XII are new.