A method for the determination of U, Th and Pu in natural water, biological materials and urine samples by electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) is described. Carbide formation was minimized using sample vaporization from a tantalum surface for U and additional use of tetrafluoromethane (Freon-23) as a gaseous modifier for Th and Pu. A prior Ca3(PO4)2 co-precipitation provided an enrichment factor of 50, yielding procedural detection limits (LOD) of 0.013, 0.029 and 0.017 pg g−1 for Th, U and Pu, respectively and corresponding absolute LODs of 0.13, 0.29 and 0.17 fg. Recovery of spikes from urine was typically 80%, whereas those from seawater, river water and biological materials averaged 99%. The accuracy of the method was validated by determination of U and Th in NIST SRM 1566b Oyster Tissue and U in a series of NRC natural water CRMs SLRS-4, CASS-4, NASS-5 and SLEW-3. Precision of determination was better than 10% at concentrations of 0.1 ng mL−1.