Green and fast: Allylation of aromatic and aliphatic thiols, by using allyl alcohols as substrates, requires only minutes at ambient temperature with a Ru catalyst (see scheme). Quantitative conversion is normal and the catalyst possesses high functional-group tolerance.The allylation of aromatic and aliphatic thiols, by using allyl alcohols as substrates, requires only minutes at ambient temperature with either a Ru(IV) catalyst, [Ru(Cp*)(eta(3)-C(3)H(5))(CH(3)CN)(2)](PF(6))(2) (2; Cp*=pentamethylcyclopentadienyl) or a combination of [Ru(Cp*)(CH(3)CN)(3)](PF(6)) and camphor sulfonic acid. Quantitative conversion is normal and the catalyst possesses high functional-group tolerance. The use of [Ru(Cp*)(CH(3)CN)(3)](PF(6)) alone affords poor results. A comparison is made to the results from catalytic runs based on the use of carbonates rather than alcohols, by using 2 as the catalyst, and it is shown that the products from the alcohols are formed faster, so there is no advantage in using a carbonate substrate. The observed branched-to-linear (b/l) ratios when using substituted alcohols decrease with time suggesting that the catalysts isomerise the products. A new methodology from which one can select the desired isomeric product is proposed. DFT calculations and NMR spectroscopic measurements, by using an arene sulfonic acid as co-catalyst, suggest that eta(6)-complexes are not relevant for the catalytic system. Moreover, the DFT results indicate that 1) any eta(6)-complexes from the acids RC(6)H(4)SO(3)H result from deprotonation of the acid, 2) complexation of the thiol, via the deprotonated sulfur atom, is preferred over complexation of the O atom of the sulfonate, RC(6)H(4)SO(3) (-) and 3) a sulfonate O-atom complex will be difficult to detect.