An amorphous AlPO4 catalyst prepared by precipitation method was subjected to pretreatment with 10 mol% water vapor at 100-300 degrees C for 15 min prior to activity tests in the dehydration of methanol to dimethyl ether (DME). The catalysts pretreated at 200-300 degrees C exhibited higher methanol conversion than the non-treated catalyst while the one treated at a lower temperature of 100 degrees C showed lower activity. Asrevealed by FT-IR and FT-Raman results, there was an increase of lattice hydroxyls on the hydrothermally treated catalysts in the form of P-OH group (weak Bronsted acid sites). Amine titration using Hammett indicators confirmed the increase of both strength and number of acid sites on these catalysts. However, hydrothermal pretreatment at 100 degrees C may result in catalyst poisoning by weakly bonded water molecules instead so that lower catalyst activity was obtained. More than 99% selectivity to DME was achieved on both non-treated and hydrothermally treated AlPO4 catalysts in this study. (C) 2010 Elsevier B.V. All rights reserved.