The list of unique zeolite crystalline phases continues to grow owing to extensive variations of the molecular additives (e.g., quaternary-ammonium and alkali-earth cations) that influence or direct the formation of three-dimensional structures. The exact properties of a molecule that enable it to direct the formation of a unique zeolite lattice are still poorly understood despite the extensive body of literature describing the synthesis of novel zeolite materials (see the review by Lobo et al.). Kubota et al.and Zones et al. have discussed the importance of hydrophobicity/hydrophilicity for organic structure-directing agents (SDA) in the synthesis of pure-silica molecular sieves. (Note: when discussing the relative hydrophobicity of a quaternary-ammonium cation we refer to its tendency to partition between the organic and aqueous layers in a biphasic mixture.) Tetrapropylammonium (TPA) has an intermediate hydrophobicity/hydrophilicity and is known to be a highly effective SDA for Si-ZSM-5. Other symmetrical ammonium cations like tetraethylammonium (TEA) or tetrapentylammonium (TPenA) are too hydrophilic or too hydrophobic, respectively, to form pure-silica zeolites under “normal” hydrothermal conditions, i.e. in the absence of alkali-metal or halide ions. However, the size of these symmetric ammonium cations also changes in the order TEA