Photoreduction of MoO3/SiO2 and MoO3/SiO2.Al2O3 catalysts in CO with a laser beam at 308 nm resulted, after addition of cyclopropane to form the active sites, in a high activity for alkene metathesis. Both catalyst systems are active for normal alkenes, e.g. propene, and for functionalized alkenes, e.g. methyl oleate, already at room temperature.