The formation of nitroso-, azo- and azoxy-aromatics depends of a relatively complex scheme involving consecutive reactions. However, all different products can be produced in good yields with palladium catalysts, using optimised kinetics, selective poisoning or alloying. These reactions show different dependencies of the reaction rate on Pd particle size and alloying. Solvents have different roles, in addition to the usual one consisting in the solubilisation of the reactants: they can change the solubility of hydrogen, compete with the reactants for adsorption at the metal surface or catalyse side reactions.