In the present work, we have studied the conversion reactions of primary and secondary C-1-C-4 alcohols over trimethyl phosphite modified ZSM-5 zeolites possessing only weak acidity and the results were compared to those of unmodified HZSM-5 catalyst. FTIR experiments on the conversion of methanol clearly showed that the formation of carbon monoxide can take place via two different reaction mechanisms depending on the amount of water present in the reaction system. We believe that the conversion of methanol to dimethyl ether and then to CO takes place even on the weak acid sites, but that the conversion of dimethyl ether to hydrocarbons requires stronger acidity. Therefore, in the conversion of small alcohols (methanol and ethanol) over the trimethyl phosphite modified ZSM-5 carried out in a flow reactor, the formation of ethers was dominant, and no further conversion was observed due to the incapability of weak acid sites to catalyze the formation of hydrocarbons. In the case of higher alcohols the production of ethers decreased significantly probably due to the steric constraints caused by the channels, and therefore the dehydration of alcohols to corresponding alkenes became dominant.