The crystal structures for a suite of substituted pollucites with the compositions CsTixAl1-xSi2O6+0.5x, 0 less than or equal to x less than or equal to 1, have been determined from Rietveld analysis of powder synchrotron XRD data. Our results indicate that the pollucite end member (CsAlSi2O6) has a tetragonal structure (space group 14(1)/a), whereas all other compositions are cubic (space group Ia3d). The increased symmetry for the titanium-substituted structures is presumably due to the incorporation of additional O2- anions (needed for compensating the charge imbalance between Ti4+ and Al3+), which effectively holds open the expanded cubic framework. In situ cooling experiments of the substituted phase CsTi0.1Al0.9Si2O6.05 reveal a displacive transformation to the tetragonal structure at similar to230 K. This transformation is tricritical in nature and is analogous to the tetragonal-to-cubic transition in pollucite on heating.