The catalytic system was applied to two related, yet more sterically hindered, compounds: crotonic acid (CA) and methacrylic acid (MA) (Table ). After the carbonylation of CA, two products may be expected: methylsuccinic anhydride (MeSA), and the 6-membered cyclic anhydride, glutaric anhydride (GA). Only MeSA is obtained when performing the reaction in MeCN under pure CO, and only in poor yields (Table , entries 1-3). In acetone, both anhydrides are formed (Table , entry 4-8). Moderate yield and conversion were obtained in acetone at 90 °C under CO/H2 pressure: 26% of cyclic anhydrides after 6 h, and 47% after 16 h (Table , entries 5 and 6). Increasing the temperature to 110 °C enhanced the activity, with a yield in cyclic anhydrides of 72% after 6 h, while the selectivity is maintained (80%, Table , entry 7, average of two runs). GA the major product when acetone is used as solvent, independently of the presence of H2 in the gas phase (Table , entry 4 vs. entries 5-8). This indicates that in acetone the isomerization of cobalt carbonyl intermediates (cobalt-alkyl complexes) is possible, consistent with previously reported results. Increasing the proportion of H2 in the gas phase from 10 vol% to 20 vol% was detrimental to the selectivity of the carbonylation, as it probably enhanced hydrogenation of the substrate (Table , entry 8). The carbonylation of MA to MeSA gave poorer results: only 7% of MeSA was detected after 6 h at 90 °C, whether under pure CO or CO/H2 (Table , entries 10-11). With the best set of conditions obtained so far (110 °C, 50 bar CO/H2) we could only enhance the amount of MeSA to 24% (Table , entry 12). Under all conditions presented here, a substantial amount of MA was hydrogenated to isobutyric acid. The competition between hydrogenation and carbonylation partly explains the poor activity of this substrate. Interestingly, when submitting CA to the best set of conditions (110 °C, 6 h, 50 bar CO/H2) in acetonitrile, similar conversion of CA is obtained but with reversed selectivity: MeSA is obtained with 53% yield, and GA is obtained with 25% yield (>99% selectivity towards anhydrides, 68:32 MeSA:GA ratio) (Table , entry 9). As previously reported, 15 the regioselectivity of the carbonylative cyclization of unsaturated acids is quite sensitive to slight modifications of the catalytic system. In our case, the modulation of the selectivity may be simply achieved by changing the reaction solvent.