High precision potassium isotope ratio measurements were made using a collision-cell equipped single focusing Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS). Interferences on 41 K from 40 ArH + were largely suppressed through collision with He gas atoms, and reaction with H 2 or D 2 gas molecules in the collision cell under optimum collision gas flow conditions. Using H 2 or D 2 as the collision gas, we distinguish charged argon–deuterium molecules (ArD + ) generated in the collision cell from argon hydride (ArH + ) generated in the plasma or in the interface region (referred to as “plasma-related AH + ” hereafter), and demonstrate, for the first time, that both plasma-related and collision cell-generated ArH + are important sources of ArH + that interfere with 41 K + in collision-cell ICP-MS instruments that use H 2 as a collision gas. The use of D 2 instead of H 2 as a reactive gas in the collision cell resulted in better overall performance in K isotope ratio measurements. By combining these mass spectrometry methods with chemical purification of K by ion exchange chromatography, we achieved an internal precision of <±0.07‰ (2 standard error) and an external reproducibility of <±0.21‰ (2 standard deviation, or 95% confidence) in the 41 K/ 39 K ratio measurement for geological and biological samples. With the improved precision, it is possible to distinguish a ∼1.3‰ variation in K isotope compositions ( 41 K/ 39 K ratios) among seawater, igneous rocks, and biological samples. The K isotope system is likely to be beneficial in providing a better understanding of potassium cycling during continental weathering and the uptake of nutrients by plants.