Abstract The isotopic composition of lanthanum has been measured with high precision using a thermal ionization mass spectrometer, equipped with a Daly collector, whose linearity was verified by measuring an isotopically certified reference material for potassium (NIST 985), whose isotopes span a wide range of isotope ratios. The abundance sensitivity of the mass spectrometer in the vicinity of the measured LaO + ion beams was examined to ensure the absence of tailing effects and interfering isotopes. The isotope fractionation of the lanthanum isotopes was estimated by reference to the isotopically certified reference material for lead (NIST 981) and the fractionation of oxygen isotopes determined from LaO + measurements. These procedures are essential because of the extremely low isotope abundance of 138La. An accurate determination of the abundance of 138La is required in order to calculate the atomic weight, and because it is the parent nuclide of two geochronometers, 138La– 138Ba and 138La– 138Ce. The magnitude of the rare odd–odd neutron-deficient isotope 138La is a key nuclide in p-process nucleosynthetic calculations. The measured isotopic composition has been corrected for isotope fractionation to give 139La/ 138La = 1125 ± 3, which gives isotope abundances for 138La of 0.000888 ± 0.000002 and 139La of 0.999112 ± 0.000002. The isotope abundances and relative atomic masses of the two isotopes give an atomic weight of La of 138.905461 ± 0.000003.