Ca3−xSrxCo4O9 with different compositions (x=0, 0.15, 0.3, and 0.45) were synthesized from stoichiometric mixtures of Co3O4, CaCO3, and SrCO3, and their unit-cell parameters were determined. The powders were mixed with silicon standard powder, and XRD patterns were collected and analyzed using TOPAS. The compounds were determined to be monoclinic with space group of P2. Unit-cell parameters changed from a=4.8329(3) Å, b=4.5727(1) Å, c=10.8360(4) Å, and β=98.093(7)° when x=0, to a=4.8432(1) Å, b=4.6100(3) Å, c=10.8668(3) Å, and β=98.110(5)° when x=0.3. An unidentified second phase was observed when x=0.45.