We have measured changes of the non-stoichiometry, $Deltadelta$, in Fe-doped nicked oxide <TEX.$(Ni_{1-x}Fe_x)_{1-delta}O$, by thermogravimetry for four iron fractions, x=0.01, 0.031, 0.057 and 0.10, and three temperatures, T=1273, 1373 and 1473 K. The obtained data can be modelled by a defect structure in which substitutional trivalent iron ions, FeNi, are compensated by cation vacancies, $V_{Ni}$", and (4:1)-clusters. These clusters consist of tetravalent interstitial iron, $Fe_i;^4$</TEX., surrounded tetrahedrally by four cation vacancies and are four times negatively charged.rged.