The effect of the chloride countercation on the mechanism of the Ru-3(CO)(12)/chloride catalyzed carbonylation of nitroarenes to carbamates has been investigated. The reason for the higher activity and selectivity obtained with tetraethylammonium chloride with respect to [PPN][Cl] is due to the higher igroscopicity of the former (only when no aniline is added) and to its ability do decompose to yield triethylamine. The role of this last compound is twofold. On one hand, it accelerates the alcoholysis of the intermediately formed diarylurea. On the other, it favors a reaction pathway that consumes aniline together with nitrobenzene, thus converting a by-product into the desired product.