Thin sheetlike crystals of the metal-deficient perovskites (H(2)AEQT)M2/3I4 [M = Bi or Sb; AEQT = 5,5"'-bis-(aminoethyl)-2,2':5',2 " :5 " ,2"'-quaterthiophene] were formed from slowly cooled ethylene glycol/2-butanol solutions containing the bismuth(III) or antimony(III) iodide and AEQT . 2HI salts. Each structure was refined in a monoclinic (C2/m) subcell, with the lattice parameters a = 39.712(13) Angstrom, b = 5.976(2) Angstrom, c = 6.043(2) Angstrom, beta = 92.238(5)degrees and Z = 2 for M = Bi and a = 39.439(7) Angstrom, b = 5.952(1) Angstrom, c = 6.031(1) Angstrom, beta = 92.245(3)degrees and Z = 2 for M = Sb. The trivalent metal cations locally adopt a distorted octahedral coordination, with M-I bond lengths ranging from 3.046(1) to 3.218(3) Angstrom (3.114 Angstrom average) for M = Bi and 3.012(1) to 3.153(2) Angstrom (3.073 Angstrom average) for M = Sb. The new organic-inorganic hybrids are the first members of a metal-deficient perovskite family consisting of (Mn+)(2/n)V(n-2)/nX42- sheets, where V represents a vacancy (generally left out of the formula) and the metal cation valence, n, is greater than 2. The organic layers in the AEQT-based organic-inorganic hybrids feature edge-to-face aromatic interactions among the rigid, rodlike quaterthiophene moieties, which may help to stabilize the unusual metal-deficient layered structures.