The carboxylation with CO of C-H bonds of pseudo-aromatic systems such as furan and thiophene, mediated by palladium acetate, can be carried out in acetic acid or methanol solvent under 1 atm CO and at low temperatures. Sodium acetate is an important cocatalyst. The reaction is highly regioselective, producing in acetic acid only the mixed anhydride between acetic acid and 2-furan or 2-thiophene carboxylic acid. Working in methanol solvent, the final products are the corresponding methyl esters; however in this solvent the main product, i.e. the methyl ester of 2-furan carboxylic acid, is contamined by small amounts of several byproducts. Yields can be increased with respect to palladium either by addition of an excess of mercury(II) acetate as cocatalyst or by carrying out the reaction by a two-step process. The first step is the formation situ of the organomercury or organothallium derivatives, using Hg(O2CMe)2, HgCl2 or Tl(O2CMe)3, followed by the second step, oxidative carbonylation, mediated by palladium acetate. Yields sometimes higher than 100% with respect to palladium are achieved, maintaining the same high regioselectivity of the ring substitution. By working under pseudo-catalytic conditions (meaning the presence of an excess of oxidant such as Cu(II) acetate) we could not achieve the high yields with respect to palladium reported by other authors working under CO pressure; in addition, the regioselectivity is negatively affected. © 1990.