For n-type (phosphorus-doped) floating-zone grown silicon samples, a hitherto unreported ESR signal was detected after the samples were doped with iron and irradiated with electrons. Analysis of the hyperfine structure and the angular dependence of the resonance peak positions of the spectrum, labelled TU6, indicated that the signal originates from a defect complex of monoclinic-I symmetry containing single phosphorus and single iron atoms. The spectrum can be described in terms of a paramagnetic system with S = 1/2 and g values greatly deviating from that of a free electron, as well as spin S = 3/2 and . The spin S = 3/2 may correspond to a positively charged iron atom (3d7 state) and, thus, to a doubly positive charge state of a TU6 related complex. The fact that the TU6 signal was detected only under strong external illumination for n-type samples also supports this assignment. Results obtained during isochronal annealing of the irradiated samples suggest the possible involvement of a divacancy in the formation of an iron–phosphorus pair in silicon.