This technical note reports for the first time the simultaneous vapor generation of 16 rare-earth elements (REEs) via reaction with sodium tetrahydroborate (THB). Significant improvement in REE vapor generation efficiency was discovered in the presence of arsenazo III (ARS). Subsequent to a critical evaluation on the impacts of various experimental variables, including the concentrations of acids, THB, and ARS, blank-limited (reagent contamination and impurity from ARS) detection limits (LODs) in the range of 0.004-0.15 μg L-1 were achieved based on three times the standard deviation of the method blank by the proposed vapor generation inductively coupled plasma mass spectrometry (ICP-MS). The nature of released REE species was studied by various approaches and identified to be REE nanoparticles and REE-ARS chelates; a dual-route mechanism for the vapor generation of REEs was thereby proposed.