The real structure of FeNbO4 was determined by a combination of electron microscopy investigations and high resolution synchrotron powder diffraction. It is shown that the order-disorder phase transition from the orthorhombic high-temperature modification (alpha-PbO2-type structure) to the monoclinic low-temperature modification (wolframite-type structure) of FeNbO4 is translationengleich and leads to a domain structure that consists of a coherent mixture of the orthorhombic and the monoclinic modification. Domains of the monoclinic phase are separated by antiphase boundaries with an antiphase vector (1/2,1/2 . 0). The real structures of eight different samples have been investigated in terms of domain size and degree of cation ordering by peak-shape analysis and the ratio between integrated intensities of split-structure and fundamental reflections. The obtained real structure is correlated with the conditions of synthesis.