The competitive formation of the zeolites ferrierite and dodecasil-3C has been studied under a variety of synthetic conditions involving HF/pyridine as a solvent together with other amines. Single crystal X-ray diffraction measurements on the siliceous zeolite ferrierite product (Si18O36. 1.9C(5)H(5)N . 0.1C(3)H(9)N, space group Pmnn, a = 18.8273(6) Angstrom, b = 14.095(1) Angstrom, c = 7.4318(7) Angstrom, V = 1972.2(3) Angstrom(3), Z = 2, R = 3.1%) reveal that it contains pyridine in both channels. Small amounts of propylamine are found in the main channel. The dodecasil-3C, by contrast, contains pyridine in the larger cavities and propylamine in the smaller ones (Si17O34. 0.8C(5)H(5)N . 0.25C(3)H(9)N, space group I42d, a = 13.639(2) Angstrom, c = 19.512(4) Angstrom, V = 3629.67(6) Angstrom(3), Z = 4, R = 6.8%). Both the synthetic observations and our computer simulations indicate that the dodecasil-3C is thermodynamically more stable than the ferrierite product, which forms under kinetic control at lower temperatures or shorter reaction times. An additional phase, identified as a propylammonium hexafluorosilicate, is also present during the course of the reaction. The propylammonium cation appears to play a dual role : acting as a pH buffer and nucleating the ferrierite phase. The dodecasil-3C phase is probably nucleated by pyridinium ions.