A partial exploration has been made of the crystallisation fields of aluminosilicate gels in presence of the mixed aqueous bases NaOH, KOH, and Me₄NOH in order to examine specific chemical effects of the mixed bases over and above those expected when each base is present separately. A wide range of zeolitic phases resulted of which four, a sodalite type, an erionite type, an offretite type, and zeolite Ω(which has no natural counterpart), were especially studied, by X-ray and electron diffraction, electron microscopy, and by thermogravimetric, differential thermal, and chemical analysis. These phases proved to be silica-rich; from the known structures and the chemical results it was also possible to locate tetramethylammonium ion positions. Reasons for the non-additive chemical effects of mixed bases in zeolite crystallisation have also been given.