AbstractFerrierite, a medium‐port molecular sieve zeolite, was synthesized as the only crystalline phase from an aqueous gel of composition 1.6 Na2O · 1.6 K2O · Al2O3· 13.5 SiO2· 130 H2O · 0.8 CO2· 2.4 (HCO2)2at 250 °C in reaction periods ranging from 40 to 95 hours. The crystallization time was reduced with seeding. Ferrierite was a metastable phase in this system and it transformed into tridymite, high sanidine and orthoclase at longer reaction periods. At 200 °C, chabazite was crystallized from the same batch composition at a very slow rate. At temperatures higher than 250 °C and in the absence of carbonates and bicarbonates in the batch mixture, tridymite, high sanidine and orthoclase crystallized directly without the formation of ferrierite. The synthetic ferrierite sorbs benzene, toluene and cyclohexane and behaves as a medium port molecular sieve. The sorption capacity of ferrierite was improved by acid treatments with no deterioration of the crystal