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.