The invention relates to a process for the preparation of 3-substituted thiophenes in which the substituent is a C.sub.4 -C.sub.18 -alkyl group. According to the invention, the preparation is carried out by allowing an alkali metal salt of a C.sub.4 -C.sub.18 -alkyl-succinic acid to react with a compound of sulfur and phosphorus, e.g. tetraphosphorus decasulfide, in an organic solvent at a temperature of 100.degree.-180.degree. C., the product obtained being 3-(C.sub.4 -C.sub.18 -alkyl)thiophene. The said reaction may be the last step in a process in which the initial material is maleic acid anhydride with which .alpha.-C.sub.4 -C.sub.18 -alkene is combined so that C.sub.4 -C.sub.18 -alkenylsuccinic acid anhydride is obtained, whereafter this is hydrogenated catalytically to produce C.sub.4 -C.sub.18 -alkylsuccinic acid anhydride. This is thereafter reacted with an alkali metal hydroxide so that an alkali salt of the C.sub.4 -C.sub.18 -alkylsuccinic acid is obtained, which salt is reacted with a compound of sulfur and phosphorus to produce 3-(C.sub.4 -C.sub.18 -alkyl)thiophene, which constitutes the final product.