This Article is focused on the study of the effect of oxygenate impurities like esters and acids in the mixed methanol and ethanol feed stream on the conversion of methanol and ethanol to light olefins. The conversion of such components at 450 degrees C over ZSM-5 was investigated. Furthermore, the effect on conversion and selectivity of adding yttrium oxide to the catalyst was studied. It was shown that the presence of relatively small amounts of esters in the alcohol feed stream has a significant effect on catalyst lifetime. These esters will decompose on the zeolite surface leading to additional methanol or ethanol (depending on the type of ester) and to a carboxylic acid. The additional methanol or ethanol will react according to the alcohol to olefins mechanism, leading to either an increase or a decrease in the propylene/ethylene (P/E) selectivity ratio, respectively. The acid that is released upon decomposition of the ester is responsible for the decrease in lifetime of the zeolite by adsorption on the zeolite active sites. Addition of metal oxides, specifically yttria, to ZSM-5 limits this decrease in catalyst lifetime. The proposed pathway is by a stronger adsorption of the acid on yttria than on the active site of the zeolite. Removal of the acid by yttria can in some cases (typically when the ester conversion is not 100% without the yttria present) lead to a higher conversion of the ester and thereby indirectly to an effect on the P/E selectivity ratio due to the additional methanol or ethanol released.