Abstract A new method was established to determine the free inorganic cadmium ion ( Cd2+) in marine bivalves using high performance liquid chromatography and inductively coupled plasma mass spectrometry ( HPLC-ICP-MS) . The free Cd2+ in shellfish was ultrasonic extracted for 40 min by 10 mmol/L Tris buffer solution ( with 0. 1 mol/L NaCl, pH=7. 5), and the separation of Cd2+ was achieved using an IonPac CS5A analytical column with an IonPac CG5A guard column. The mobile phase consisted of 50 mmol/L C2 H2 O4 and 95 mmol/L LiOH. The new method had a good linear relationship with the correlation coefficient of 0. 999 and the standard recoveries of Cd2+ were all above 84. 6%. The free Cd2+ content in several kinds of marine bivalves was determined using the method and the results showed that the percentage of free inorganic Cd2+ to the total Cd content in samples with high Cd content was higher than those with low Cd content.