A series of phosphorus modified HZSM-5 (SiO2/Al2O3= 25, P/Al = 0.5, 1.0, 1.5, 2.0) samples were prepared and tested in the isomerization of styrene oxide to phenylacetaldehyde under a gas-phase atmosphere free of solvents. The strength and concentration of Bronsted acid sites decreased with increased phosphorus content, while all samples exhibited initial conversions of styrene oxide greater than 99%. External acid sites were gradually deactivated with phosphorus addition, improving the phenylacetaldehyde selectivity by suppressing its trimerization which occurred at external acid sites. When the P/Al ratio <= 1, catalyst lifetimes extended because of the decrease in strong acid strength. Hard coke, which preferentially formed on strong acid sites, was considerably inhibited in the presence of phosphorus. (C) 2014 Elsevier B.V. All rights reserved.