Measurements of long-lived radionuclides in radioactive waste are essential for assessing its environmental impact. These data, which are reported to the government and authorized institutions, have become requested to be traceable to the SI to ensure the universal reliability. Among such long-lived radionuclides, ¹³⁵Cs has attracted attention due to its high fission yield, comparable to that of ¹³⁷Cs as well as the high solubility and mobility of Cs ions in environmental aquatic system. However, the lack of a commercially available certified reference material (CRM) for ¹³⁵Cs poses a challenge for the establishment of SI traceability. In this study, the mass fractions of Cs isotopes (¹³³Cs, ¹³⁵Cs and ¹³⁷Cs) in the ¹³⁷Cs solution were determined traceably to the SI using standards of Cs(¹³³Cs) and ¹³⁷Cs, along with the isotopic composition, isotope ratios, and the Cs molar mass by ICP-MS/MS.