The relative stability of the two lowest singlet states of AlP3 (C-2V and C-S symmetries) is studied using high-level ab initio calculations. Extrapolation to the infinite basis set limit using CCSD(T)/cc-pVXZ (X = 2, 3, 4 and 5) give the energy difference as 1.6 kcal/mol, with the C-2V structure lying lowest. Triple excitation contribution to the electron correlation effect and large basis sets are found to be very important. The locations of the two states on the potential energy hyper-surface are obtained to show that they represent well-defined and stable isomers. (c) 2005 Elsevier B.V. All rights reserved.