A novel heterogeneously catalyzed solvent-free process for the acylation of anisole with acetic anhydride is presented. Large pore size ion exchange resins comprising sulfonic acid groups as active centres were found unexpectedly to be suitable catalysts for this reaction. Solid state NMR-measurements and differential thermo gravimetric analyses were carried out to demonstrate that 1-(4-methoxyphenyl)ethanone and other reaction products were incorporated into the catalyst matrix of small pore size ion exchange resins causing fast deactivation due to a blockage of the active centers. In contrast, ion exchange resins with larger pore sizes, however, did not show such a behavior. In this case by running the reaction in a continuous plug flow reactor more than 20% conversion of anisole and 94% selectivity for 1-(4-methoxy-phenyl)- ethanone were achieved without any catalyst deactivation for more than 1000 h time on stream (TOS) under the chosen reaction conditions of 110 °C, weight hourly space velocity (WHSV) = 0.2 h^-1 and a molar ratio of anisole to acetic anhydride of 1:1.2.