The reaction of ethyl diazoacetate with styrene is catalysed by Cu-II-exchanged clays and zeolites; the trans/cis Selectivity strongly depends on the nature of the catalyst and when using Cu-II-K10 montmorillonite the normal selectivity in solution is reversed and the cis-cyclopropane is the major product. The tendency of this catalyst to increase the amount of the product less favoured in solution is confirmed in other reactions.