Although it was reported that carboplatin, a second-generation platinum agent, exhibits an anti-cancer effect after the structure is changed from carboplatin to cisplatin, how this change takes place remains unclear. In this study, the stability of carboplatin to pH and Cl- concentration was examined by HPLC using πNAP column. In the absence of sodium chloride, carboplatin did not change to cisplatin. However, in the presence of sodium chloride at pH 3.0 and 6.0, carboplatin changed to cisplatin after 8 hr and carboplatin changed to 0.33 mmol cisplatin at 24 hr in the presence of sodium chloride at pH 3.0. The result of this study shows that when carboplatin was mixed with an infusion solution of inorganic salts including chloride, the solution should be infused within 8 hr.