Phosphate-modified ZSM-5 zeolites were studied by standard characterization techniques and solid-state nuclear magnetic resonance spectroscopy, and in the methanol-to-olefin (MTO) conversion. Considering the physicochemical properties of the ZSM-5 zeolites, the most important effects of the phosphate modification are a deposition of polyphosphates in the pore system and a decrease of the acid site density, but not of the acid site strength. The significant increase in the selectivity to C-2-C-4 alkenes and the decrease of C5+ formation in the MTO reaction for a phosphate coverage of about 5wt% are explained by extra-framework phosphate species near the crossing intersections of the ZSM-5 pore system, which hinder the formation of large intermediates and reaction products.