Thermal helium desorption spectrometry (THDS) has been employed to study the nucleation and growth of helium defect agglomerates in niobium and vanadium. An important difference from other body-centered-cubic metals was the temperature at which helium was released from monovacancy type defects. In Mo and W the release takes place at 0.40 T m ( T m = melting temperature) but in V and Nb at 0.25 T m . This and other observations led to the conclusion that in nearly all interactions of helium with radiation-induced defects, impurities, mainly oxygen, are involved. Impurity levels of 20–30 appm are sufficient to completely suppress the type of helium behaviour that was found in Mo and W. Oxygen decorates the helium-vacancy complexes and makes the helium desorption temperature lower, but stabilizes the small defect complexes up to a temperature of about 700 K; beyond this temperature, nucleation of defect clusters is considerably reduced. Oxygen decoration has been observed to reduce the sink strength of the defects for helium trapping.