Abstract: Thallium leucite, TlAlSi₂O₆, has been synthesized at 450°C for 7 days, under ambient conditions, by the transformation of dehydrated analcime NaAlSi₂O₆ in the presence of excess TlCl. This substitution of Tl for Na leads to confirmation of a thallium-leucite pseudomorph after analcime. Their optical properties, X-ray powder diffraction patterns, electron microprobe analysis, infrared spectra, and X-ray photoelectron spectroscopy have characterized the synthetic Tl-leucites. The IR spectra show that the mid-IR modes T-O stretching and T-O-T bending vibrations for TlAlSi₂O₆ are more resemblant of those for analcime than for leucite, KAlSi₂O₆. This resemblance implies that Tl cation enters the W-site rather than the S-site in the analcime structure: Na (S) + H₂O (W) ⇌ □ + K (leucite) ⇌ □ + Tl (Tl- leucite), where □ represents an S-site vacancy. The mechanism of this substitution is supported by the crystal chemical constraints: inasmuch as the S-site is smaller than the W-site, Tl⁺ cations being larger than Na⁺ plainly prefer the latter site to the former. One inference from the binding energy for Tl⁺ by XPS is that Tl⁺ occupies the extra-framework site in synthetic leucite pseudomorph, rather than the smaller tetrahedral site. The difference in Al/Si disordering between analcime and leucite and the nonstoichiometry due to the solid solution of the □Si₃O₆ component into the leucite structure may provide a fundamental insight into understanding why TlAlSi₂O₆ deviates from the trend defined by K-, Rb- and CsAlSi₂O₆ leucite series on the a-c parameter diagram, inasmuch as these three cations in the leucite structure occupy the W-sites. Finally, synthesis of TlAlSi₂O₆ leucite has an implication for the existence of other polymorphs due to different degrees of Al/Si disordering, except for high- and low-temperature leucites already known: natural leucites crystallized directly through igneous processes are different from those formed by substitution of K for Na in analcimes.