Abstract The effects of helium on macroscopic mechanical properties in vanadium alloys have been studied rather extensively. However, the evolution processes of helium-defect complexes, especially during the early stage, is not clearly understood. This is mainly because of the complexity of helium behavior due to the large amount of interstitial impurities (C, N, O, etc.) in vanadium. THDS (thermal helium desorption spectrometry) was performed to examine the nature and behavior of helium-defect complexes. The observed desorption peaks were assumed to be concerned with vacancy type defects. In vanadium, most of vacancies are decorated by interstitial impurities, and implanted helium produces He n V n X-type defects (X = C, N, O). The desorption peaks of 570 K, 690 K and 940 K were assigned as He n OV, He n OV 2 and He n OV 4, respectively. The population of these peaks increased with oxygen concentration. On the other hand, some peaks which were independent of impurity concentration were deduced as impurity free defect clusters such as He n V n .