Aryl- and allylsilanes are carboxylated with CO2 with the aid of aluminum-based Lewis acids, to give aromatic and beta, gamma-unsaturated carboxylic acids in fair to good yields, respectively. Formation of the former is rationalized by an aromatic electrophilic substitution by CO2 activated by the Lewis acid, while the latter a nucleophilic addition of in situ-generated allylaluminum species to CO2.