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The present study concerns X-ray characterisation of the microstructures of the martensites of Fe–23Ni–3.6Mn alloy, transformed isothermally at low temperature. Along with the austenized powder and bulk forms of the alloy, coldworked powders have also been analysed. The methodology adopted is Rietveld's whole profile fitting technique which incorporates correction for preferred orientation of the crystallites. The results reveal important information on the crystallite (domain) sizes, residual microstrains, preferred orientation, stacking and compound fault probabilities, dislocation density etc., for both the austenite and martensite phases of the alloy. The martensite has smaller crystallite sizes and larger microstrain values, both of which are isotropic in nature for the transformed bulk and the austenized powders but anisotropic for the coldworked powders. The transformed matrix revealed high percentage of martensites in coldworked powder and bulk whereas annealed powder revealed about 8% volume fraction of martensites. The dislocation density values evaluated from the respective crystallite sizes and r.m.s. strain values are high (∼1011 cm/cm3) in the martensitic phase by almost an order of magnitude compared to their respective austenite phase. The coldworked powder reveals high propensity of faulting. The hardness value of transformed bulk (191 kg/mm2) is more than double the value for the austenite bulk (95 kg/mm2). The results have been compared and correlated with those in two previous studies on the same alloy system having 3.3 and 3.8 mass% of Mn.