The behaviour of rare earth elements (REEs) ions within a dynamic reaction cell (DRC) pressurized with oxygen was investigated, in order to explore their determination at m/z +16 by inductively coupled plasma mass spectrometry (ICP-MS). Under the optimized conditions, the conversion of REEs to molecular monoxide ions was nearly quantitative (>96%), with the exception of Tm (78%), Eu (11%) and Yb (6%). Moreover, the formation of dioxide ions was generally lower than 1%. According to these results, a new method for the determination of REEs in digests from geological samples was developed. After microwave-assisted acid digestion, the solutions were directly analyzed by ICP-MS, determining REEs at m/z +16. The interferences due to dioxide and un-reacted ions were carefully evaluated by taking into account the actual concentration of REEs in sediment samples. In addition, the possible interferences due to other constituents of sediments (e.g. barium) were considered. The accurate determination of Y, La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er and Tm was achieved, as demonstrated by the analysis of both certified reference materials and Antarctic marine sediment samples, using ICP sector field mass spectrometry (ICP-SFMS) for comparison. On the other hand, Eu, Gd, Yb and Lu could not be measured by the O-atom addition approach and their separate determination in the standard mode using mathematical correction is hence necessary to obtain the complete REE pattern. The quantification limits (10 times the standard deviation of 10 procedural blanks) were at least two orders of magnitude lower than the REEs concentrations in sediment samples. Precision of the procedure ranged from 1.6% to 6.4%.