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)