A high resolution inductively coupled plasma mass spectrometric (HR-ICP-MS) method for the determination of plutonium isotopes and the ²⁴⁰ Pu/ ²³⁹ Pu isotope ratio utilising ultrasonic nebulisation was developed. Total plutonium concentrations ( ^239+240 Pu) measured in environmental samples by this HR-ICP-MS method were in good agreement with data obtained from α-spectrometry. Quantification was performed by both external calibration and isotope dilution and the best agreement was found by applying isotope dilution. Detection limits of 5, 1 and 1 fg ml ^–1 were found for ²³⁹ Pu, ²⁴⁰ Pu and ²⁴² Pu, respectively. These represent total amounts of 50, 10 and 10 fg or 0.1, 0.08 and 0.002 mBq of the three isotopes in a 10 ml sample volume. The precision (RSD) on the measurement of the ²⁴⁰ Pu/ ²³⁹ Pu ratio in environmental samples was approximately 2%, which is close to the theoretical precision (Poisson statistics). The influence of dwell time, number of sweeps and sample uptake rate on the measurement precision of the ²⁴⁰ Pu/ ²³⁹ Pu isotope ratio was investigated and the parameters were optimised using a 2 ³ experimental design (a central composite design followed by a ridge analysis). The accuracy of the isotope ratio measurement was evaluated by comparing results from this HR-ICP-MS method with results from high resolution α-spectrometry and spectral deconvolution. Good agreement was found between the two techniques.