Ab initio theoretical methods have been used to study small aluminum-phosphorus compounds. Geometries, relative energies, and harmonic vibrational frequencies of isomers of the formulas AlPH2, AlPH3, and AlPH4 and the H3Al-PH3 adduct have been determined at the self-consistent-field level (SCF), by the single- and double-excitation configuration interaction (CISD), and by the single- and double-excitation coupled cluster (CCSD) method. The lowest energy isomers for AlPH2, AlPH3, and AlPH4 are Al-PH2, H2Al-PH, and H2Al-PH2, respectively. The HAl-PH isomer is planar and has a short Al-P bond. An aluminum atom forms a weak complex with phosphine, and the barrier for insertion of Al into PH3 is estimated to be 12.0 kcal mol(-1). Various conformations of the H2Al-PH2 molecule are investigated to give estimates of the energy of the pi donor-acceptor bond. The energy of complexation for AlH3 and PH3 is obtained using triple-zeta-plus double-polarization function basis sets and the CCSD level. The theoretical energy of H-2 loss is also obtained for H3Al-PH3 and H2Al-PH2.