The determination of Pt by ICP-MS in environmental and biological samples is well documented and generally performed after dissolution in dilute HNO 3.On the other hand, I is poorly ionised in the plasma and, at low pH, memory effects and instability arise from the formation of potentially volatile species, such as I 2 and HI, depending on the oxidation state of I. In order to investigate the role of iodo ligands in the design of Pt anticancer complexes, we have optimised conditions for the simultaneous determination of Pt and I. Standards and samples were diluted in 10 mM KOH and improved ion extraction into the quadrupole was achieved by means of an additional pump (S-Option@), leading to about a 2-fold increased sensitivity. The limits of detection in water were 10 and 0.6 ng l−1 for I and Pt, respectively, but increased to 23 ng l−1 for I and 2.2 ng l−1for Pt in KOH (10 mM ). The analysis of certified reference materials yielded the following results: 0.84±0.05 mg g−1 I (certified value: 0.81±0.05 mg g−1) in BCR CRM 063R 'Skimmed Milk Powder', and 0.121±0.006 mg ml−1 Pt (indicative value: 0.12 mg ml−1) in NIST SRM 2670 'Toxic Metals in Urine'. Pt5I ratios ranging from 0.240 to 1.035 were measured with an accuracy of 101.3±2.4%. The determination of the Pt5I ratio in the low M r fraction of reaction mixtures of diiodo Pt complexes and human albumin provided evidence for the release of iodide and for different kinetics for the reactions of diiodo Pt() and Pt() complexes.