Plasma oxidation for ¹⁴ C sampling utilizes low-pressure (133 Pa), low-energy (<50 W), and low- temperature (<50°C) Ar- and O 2 -plasmas generating CO 2 for AMS dating. O 2 -plasmas on empty chambers remove organic contamination. When clean, a new specimen is inserted and Ar-plasmas dislodge adsorbed atmospheric CO2 from surfaces. Finally, O 2 -plasmas oxidize organic materials to CO 2 for AMS analysis. During some Ar-plasmas we observed anomalous pressure increases and unexpectedly high CO2. The residual gas analysis detected water, hydrogen, and oxygen species with Ar and CO 2 indicating water plasmas that produced excited oxygen species that prematurely oxidized specimen organic matter. The evolution of excess CO2 during Ar cleaning compromises the ability to affirm that atmospheric CO2 was removed. Standards, TIRI Belfast Pine and VIRI I Whalebone, were dated to determine whether water-induced oxidation was a confounding influence in dating. TIRI wood was sampled twice, once a water-soaked specimen in an Ar plasma and once with water-vapor plasma only. The TIRI dates agreed with six earlier dates on usual specimens. A colloidal extract from VIRI I whalebone was also sampled and dated twice using both water–plasma oxidation in an Ar-plasma and in an O 2 -plasma. The dating agreement suggests that water plasmas do not pose undue risks of contamination.