The relative abundance of the two lowest and energetically close singlet states of AlP3 (C-2V and C-S symmetries) is studied using second-order Moller-Plesset perturbation theory, MP2/aug-cc-pVTZ, and density-functional theory, B3LYP/aug-cc-pVTZ, calculations. The C-2V <-> C-S barrier is calculated to be around 30 kcal/mol precluding possible interconversion. Gibbs free energy calculations indicate that the C-2V form is more stable for all temperatures considered (80 K < T < 320 K). For temperatures below 150 K (supersonic expansion conditions) the relative abundance of the C-2V is larger than 90% indicating that upon cold formation experimental results will refer almost exclusively to the C-2V isomer. (C) 2007 Elsevier B.V. All rights reserved.