Abstract Investigations into the use of hydrogen addition in the nebulizer gas of an argon inductively coupled plasma (ICP) in an ICP-mass spectrometer for the possible reduction of polyatomic interferences are reported. Hydrogen was found to worsen interferences such as ArO + and ArCl + with respect to an all-argon system. In contrast to these argon-based interferences the levels of MO + were found to be reduced by approximately a factor of 10 with the addition of hydrogen. The hydrogen-argon system was optimized using a simplex procedure. At the defined, optimum the level of CeO + compared to that of Ce + was approximately 0.5%. Detection limits for Gd in 10 μg cm −3 La were 15.9 ng cm −3 in the hydrogen-argon system compared to 2000 ng cm −3 in the all-argon system. These conditions were also used for the analysis of the CRM Bastnasite mill feed ore. IGS 40, using slurry nebulization. The determination of Sm was made possible and the determination of Gd slightly improved by the hydrogen addition. It was concluded that hydrogen addition in an appropriately optimized system offers improvements in heavy rare earth element (REE) determination, in the presence of their lighter counterparts, provided the concentration of the light REE does not exceed μg cm −3 levels.