The indium-induced Si(001)-(4 X 3) reconstruction has been investigated by surface X-ray diffraction (SXRD) measurements with synchrotron radiation and scanning tunneling microscopy (STM). The Patterson function analysis enables us to exclude In dimers as a structural element in this reconstruction. We present a new structural model which includes 6 In atoms threefold coordinated to Si atoms and 5 displaced Si atoms per unit cell. Relaxations down to the sixth layer were determined. 'Trimers' made up of In-Si-In atoms are a key structural element. (C) 1998 Elsevier Science B.V.