S‐parameter positron beam measurements have been done on several kinds ofa‐Si: Kr‐sputtereda‐Si, PECVDa‐Si, MeV ion beam amorphized Si anda‐Si grown in an MBE‐system at a low deposition temperature. Kr sputtereda‐Si becomes denser for higher Kr concentration. PECVDa‐Si:H contains micro‐cavities with a size depending on growth temperature. MeV ion beam amorphized Si contains 1.2 at. % small vacancies, which decreases upon annealing (relaxation) to 0.4 at. %. This effect can be mimicked by H‐implantation and subsequent annealing, showing that at least some of the dangling bonds ina‐Si are located at these vacancy‐type defects. Finally positron measurements show that MBE‐system growna‐Si contains large open‐volume defects. The positron annihilation data are supplemented by data from some other techniques.