Temperature dependency of the average positron lifetime has been investigated for n?type float?zone silicon, n?FZ?Si(P), subjected to irradiation with 0.9 MeV electrons at RT. In the course of the isochronal annealing a new defect?related temperature?dependent pattern of the positron lifetime spectra has been revealed. Beyond the well known intervals of isochronal annealing of acceptor?like defects such as E?centers, divacancies and A?centers, the positron annihilation at the vacancy defects has been observed in the course of the isochronal annealing from ? 320 °C up to the limit of reliable detecting of the defect?related positron annihilation lifetime at ? 500 °C. These data correlate with the ones of recovery of the concentration of the charge carriers and their mobility which is found to continue in the course of annealing to ? 570 °C; the annealing is accomplished at ?650 °C. A thermally stable complex consisting of the open vacancy volume and the phosphorus impurity atom, Vop?P, is suggested as a possible candidate for interpreting the data obtained by the positron annihilation lifetime spectroscopy. An extended couple of semi?vacancies, 2Vs?ext, as well as a relaxed inwards a couple of vacancies, 2Vinw, are suggested as the open vacancy volume Vop to be probed with the positron. It is argued that a high thermal stability of the Vs?ext PVs?ext (or VinwPVinw.) configuration is contributed by the efficiency of PSi5 bonding. (© 2016 WILEY?VCH Verlag GmbH & Co. KGaA, Weinheim)