An elegant method is described for the synthesis of ZSM-22. By controlled addition of seeds and applying stirring during the crystallization, the phase purity and the crystal size can be controlled. The H-ZSM-22 is tested in the dense phase oligomerization of propene at 180-250 degrees C and 70 bar. It is found that adding small amounts of water significantly increases the activity and decreases the deactivation of the catalyst. Secondly, it is shown that catalyst activity is higher after calcination at 400 degrees C versus 550 degrees C and that smaller crystal sizes positively affect the catalyst activity. Both observations are ascribed to the presence of active sites located near the zeolite outer surface accounting for a major part of the catalytic activity.