The present invention provides a process for preparing ethylene and/or propylene, wherein an oxygenate feedstock is contacted with a molecular sieve-comprising catalyst at a temperature in the range of from 350 to 500° C. to obtain a reactor effluent comprising ethylene and/or propylene and the oxygenate feedstock is contacted with the catalyst in a riser reactor having a reactor wall defining a flow trajectory towards a downstream outlet for reactor effluent, wherein at least oxygenate feedstock and catalyst are provided at one or more upstream inlets of the riser reactor and wherein a water quench medium is admitted to the riser reactor at one or more of locations along the length of the flow trajectory through a plurality of inlets distributed along the periphery of the reactor wall. The invention further provides a reaction system suitable for preparing ethylene and propylene.