Two linear ((1)Sigma(+) and (1)Pi) and three bent (1(1)A', 2(1)A', and 1(1)A") lowest-lying electronic singlet states of AlOH have been systematically investigated employing ab initio self-consistent-field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], and CCSD with iterative partial triple excitations (CCSDT-3 and CC3) quantum mechanical methods with basis sets up to augmented correlation consistent polarized valence quadruple zeta (aug-cc-pVQZ). The linear (1)Sigma(+) state is found to be a remarkably low-energy transition state between the two equivalent bent 1(1)A' structures, while the linear (1)Pi state is a second-order saddle point, which leads to the bent 2(1)A' and 1(1)A" states. The bent ground ((X) over tilde (1)A') state of AlOH is predicted to have a bond angle of 157degrees at the aug-cc-pVQZ CC3 level of theory and is classified as a quasilinear molecule, confirming previous studies. Employing the equation-of-motion coupled cluster method, the first singlet excited state ((A) over tilde (1)A') is predicted to have a bond angle of 110degrees and to lie 114 kcal/mol (39900 cm(-1), 4.94 eV) above the ground state, whereas the second singlet excited state ((B) over tilde (1)A") is predicted to have a bond angle of 116degrees and to be located 119 kcal/mol (41700 cm(-1), 5.17 eV) above the ground state. These theoretical energy separations are in excellent agreement with the experimental values T-0 ((A) over tilde (1)A')=114.57 kcal/mol (40073 cm(-1), 4.968 eV) and T-0 ((B) over tilde (1)A")=119.36 kcal/mol (41747 cm(-1), 5.176 eV). The barriers to linearity for the two bent singlet excited states are determined to be 11.6 kcal/mol for the (A) over tilde (1)A' state and 6.2 kcal/mol for the (B) over tilde (1)A" state. (C) 2003 American Institute of Physics.