The present invention provides a process for preparing ethylene and/or propylene, comprising the steps of: a) providing an oxygenate-comprising feedstock; b) contacting the oxygenate-comprising feedstock with a molecular sieve-comprising catalyst at a temperature in the range of from 450 to 700° C. and converting at least part of the oxygenate into an olefinic product comprising ethylene and/or propylene; and c) retrieving the olefinic product, wherein the oxygenate-comprising feedstock comprises in the range of from 1 to 97 wt % of at least one tert-alkyl ether selected from the group MTBE, ETBE, TAME and TAEE, based on the weight of the oxygenates in the oxygenate-comprising feedstock, and further comprises methanol and/or DME.