The stability issue is very critical for organic hybrid perovskite CH3NH3PbI3 to be used in solar cell applications; whereas CsPbI3 is reported to be more stable than that of CH3NH3PbI3. In the present work, we attempt to substitute Cs+ in the organic hybrid perovskite CH3NH3PbI3 matrix to form Csx(CH3NH3)1-xPbI3 with 0 ≤ x ≤ 0.4 in ambient conditions i.e., at room temperature and in air via solid state reaction route. The structural studies reveal presence of both CH3NH3PbI3 (tetragonal, I4/mcm) and CsPbI3 (orthorhombic, Pnma) phases for x > 0. It is appearing that partial solid solution formation has occurred up to x = 0.2 but for x > 0.2, the composite formation dominates. The optical band gap is slightly increased by ∼0.02 eV with substitution. These compounds keep the basic feature of parent CH3NH3PbI3 along with stability of CsPbI3. •Composite formation of Csx (CH3NH3) (1-x)PbI3 processed via cold sintering method.•Composite formation is studied using structural and optical properties.•Further, this composite formation has also been verified thermodynamically.