A novel method is described for the preparation of offretite/erionite type zeolites without use of nitrogen-containing templates. It is based on the introduction of a certain amount of potassium ions into a synthesis system leading to the formation of an omega type zeolite. The synthesized product is characterized by X-ray diffraction and scanning electron microscopy and is identified as a single phase offretite. In addition, both sorption and ion exchange properties of the product are studied, and the results indicate that the large pore openings of offretite are effectively blocked. It is suggested that a small amount of intergrowth of erionite may be present in the offretite crystals, and this is responsible for the blockade.