The crystal structure of Al3O3.5C0.5 (Z = 3) has been characterized by X-ray powder diffraction (XRPD), transmission electron microscopy, and electron probe microanalysis. The title compound is trigonal with space group R (3) over barm (centrosymmetric) and hexagonal unit-cell dimensions a = 0.29588(1) nm, c = 2.84080(7) nm, and V = 0.21538(1) nm(3). The initial structural model was determined by the charge-flipping method and subsequently refined by the Rietveld method. The final structural model showed the positional disordering of one of the two types of Al sites. The maximum-entropy method-based pattern fitting (MPF) method was used to confirm the validity of the split-atom model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The reliability indices calculated from the MPF were R-wp = 4.03%, S (= R-wp/R-e) = 1.17, R-p = 3.08%, R-B = 0.82%, and R-F = 0.72%. The crystal was composed of antiphase domains, suggesting the occurrence of the high-low phase transition during the cooling process. The transition would be accompanied by the loss of unit-lattice translation, and hence the disordered structural model determined by XRPD might be of the average structure of the low-temperature phase.