Abstract: Synthesis of polyoxymethylene dimethyl ethers was carried out on HMCM-22, Al-YNU-1 and MOR zeolites by the condensation of trioxane and methanol or methylal. The crystal structures and surface acidity of the catalysts were characterized by XRD, SEM and NH₃-TPD. It was found that DMM_2~8 selectivity was influenced by the acid strength and density of zeolites and reaction conditions. HMCM-22 showed much higher selectivity to DMM_2~8 than Al-YNU-1 and MOR. Under the optimal conditions, the DMM_2~8 selectivity reached 65.1% and 90.6% in ME/TRIox and DMM/TRIox systems respectively. It was shown that the moderately strong acid sites were probably responsible for the formation of long chain DMM_n, while the strong acid sites werefor DMM.