A quaternary oxycarbide, [ Al 16.77 ( 5 ) Si 1.23 ( 5 ) ] Σ 18 [ O 3.04 ( 9 ) C 10.96 ( 9 ) ] Σ 14 , has been for the first time discovered in the Al–Si–O–C system. The crystal structure was characterized by X-ray powder diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX). The atom ratios [Al:Si] were determined by EDX, and the initial structural model was derived by the direct methods. The structural parameters as well as the atom ratios [O:C] were determined by the Rietveld method. The crystal is monoclinic (space group C2/ m, Z=1) with lattice dimensions a=0.57404(1) nm, b=0.331435(5) nm, c=1.92410(2) nm, β=90.036(1)° and V=0.366076(9) nm 3. The final structural model showed the positional disordering of Al/Si sites. The validity of the split-atom model was verified by the three-dimensional electron density distribution, the structural bias of which was reduced as much as possible using the maximum-entropy methods-based pattern fitting (MPF). The reliability indices calculated from the MPF were R wp=4.20% ( S=1.14), R p=3.09%, R B=0.92% and R F =1.05%. The crystal was an inversion twin with nearly the same twin fraction.