The submillimetre-wave spectrum of the molecular ions N15NH+, 15NNH+, N15ND+, and 15NND+ have been investigated in the laboratory using a source-modulation microwave spectrometer equipped with a negative glow discharge cell. The diazenylium ion was produced in a Ar/N2/H2(D2) discharge plasma and the 15N-containing isotopologues were observed in natural abundance. Six new rotational transitions for the protonated species and seven for the deuterated ones were accurately measured in the frequency range 270−760 GHz. These new laboratory measurements of the rare isotopologues of N2H+ provide very precise rest frequencies at millimetre and submillimetre wavelengths useful for the radioastronomical identification of their rotational lines in the ISM.