The reaction of alkenes (allyl alcohol, styrene and C6 alkenes) with formaldehyde was efficiently performed by using Rh precatalysts formed <em>in situ</em> by the addition of triphenylphosphine (PPh<sub>3</sub>), 1,2-bis(diphenylphosphino)ethane (dppe) or 1,1,1-tris(diphenylphosphinomethyl) ethane (triphos) to the complex Rh(acac)(CO)<sub>2</sub> at 130ºC in 1,4-dioxane, yielding their corresponding aldehydes; the best catalytic system was Rh(acac)(CO)<sub>2</sub>/2dppe, which generates the cationic complex [Rh(k<sup>2</sup>-P,P-dppe)<sub>2</sub>]<sup>+</sup>. However, the reaction of phenylacetylene with formaldehyde under the same reaction conditions generated styrene, which was found to be the product of transfer hydrogenation from formaldehyde.