Cesium tungsten bronzes (CsO32WO3) have attracted much attention as a near-infrared absorbing material. We report the successful synthesis of highly crystalline and high purity CsO32WO3 nanoparticles through a spray pyrolysis route. Careful analyses disclosed the presence of cationic defects, that is, a tungsten deficiency and insufficient Cs doping in the CsO32WO3 nanoparticles. These cationic defects can be controlled by facile heat treatment in a mildly reducing atmosphere. In particular, we clarify that the tungsten deficiency is a key factor among the cationic defects to obtain high near-infrared absorption properties. Furthermore, this study clearly demonstrates the precise tunability of the optical properties by means of the lattice constants of the CsO32WO3 crystal. The realized range of lattice constants is significantly wider than those previously reported. These findings should contribute to the engineering of CsO32WO3 structure and properties.