Cs-deficient pollucite compounds with the general composition Cs1-xAl1-xSi2+xO6 and a cubic pollucite structure at room temperature (RT) were prepared by a multistep solid-state reaction. The single phase of the products was confirmed by powder X-ray diffraction (XRD) analysis at RT. Structural phase transition was influenced by the Cs content of compounds of the Cs1-xAl1-xSi2+xO6 series. Cs0.8Al0.8Si2.2O6 and Cs0.75Al0.75Si2.25O6 were cubic at 123 K from the XRD patterns and did not undergo structural phase transition. The lattice thermal expansion behavior of the Cs-deficient pollucite compounds was investigated by low-temperature and high-temperature XRD analyses in the temperature range of 123-1173 K and was found to show a gradual decrease with decreasing Cs content of Cs1-xAl1-xSi2+xO6 series. The variation in unit cell volume with temperature for each composition showed that Cs0.8Al0.8Si2.2O6 and Cs0.75Al0.75Si2.25O6 had significantly smaller degrees of lattice thermal expansion than CsAlSi2O6 and Cs0.9Al0.9Si2.1O6. CsAlSi2O6, Cs0.9Al0.9Si2.1O6, Cs0.8Al0.8Si2.2O6, and Cs0.75Al0.75Si2.25O6 showed linear coefficients of thermal expansion of 2.7 x 10(-6), 2.2 x 10(-6), 1.5 x 10(-6), and 1.1 x 10(-6) K-1, respectively, which are attributed to not only an increase in space ratio but also a decrease in Al/Si ratio in the framework in the unit cell.