The isotope ratios of cerium and lanthanum are determined precisely by a surface ionization mass spectrometer. ¹³⁸Ce/¹⁴²Ce and ¹⁴⁰Ce/¹⁴²Ce ratios for Ce standard solution are 0.0225762± 0.0000014 (2σ) and 7.9471±0.0003 (2σ), normalized to ¹³⁶Ce/¹⁴²Ce=0.01688, respectively. ¹³⁸La/¹³⁹La ratio is 0.0009025±0.0000005 (2σ). The oxygen isotope ratios in measurements of REE as oxide ions are 0.0003916±0.0000014 (2σ) and 0.002129±0.000010 (2σ) for ¹⁷O/¹⁶O and ¹⁸O/¹⁶O. The isotopic abundances (%) of ¹³⁶Ce, ¹³⁸Ce, ¹⁴⁰Ce and ¹⁴²Ce are calculated to be 0.1878, 0.2512, 88.433 and 11.128, respectively. The isotopic abundances (%) of ¹³⁸La and ¹³⁹La are calculated to be 0.09016 and 99.9098. These isotopic abundances obtained here give the following relation:
(¹³⁸La/¹⁴²Ce)_ATOMIC=0.008173×(La/Ce)_WEIGHT.
These precise isotopic abundances on Ce and La are fundamental values in a La-Ce geochronometer.