A double focusing inductively coupled plasma mass spectrometer with an ultrasonic nebulizer and a desolvation unit was used to measure lead-206 lead-207, lead-206 lead-208 and lead-207 lead-208 ratios in lead standard SRM981, diluted to total lead concentrations of 1 to 1000 ng per litre. At lead concentrations down to 10 ng per litre, it was possible to determine isotope ratios with sufficient precision, accuracy and reproducibility for many environmental studies. At 1 ng per litre, the procedural blanks were a problem. The method was used to obtain lead isotope ratios for synthetic (SRM1643d) and natural (SLRS-3) standard waters. Reproducibility was excellent for the synthetic water, but lower for the natural water, probably due to matrix effects. Isotope measurements at the sub-ng per litre level will require further work to reduce procedural blanks. In particular, instrument contamination and memory effects for lead will need to be solved.