Millimeter wave rotational spectroscopy and ab initio calculations are used to explore the potential energy surface of LiOH and LiOD with particular emphasis on the bending states and bending potential. New measurements extend the observed rotational lines to J=7<--6 for LiOH and J=8<--7 for LiOD for all bending vibrational states up to (03(3)0). Rotation-vibration energy levels, geometric expectation values, and dipole moments are calculated using extensive high-level ab initio three-dimensional potential energy and dipole moment surfaces. Agreement between calculation and experiment is superb, with predicted B-v values typically within 0.3%, D values within 0.2%, q(l) values within 0.7%, and dipole moments within 0.9% of experiment. Shifts in B-v values with vibration and isotopic substitution are also well predicted. A combined theoretical and experimental structural analysis establishes the linear equilibrium structure with r(e)(Li-O)=1.5776(4) A and r(e)(O-H)=0.949(2) A. Predicted fundamental vibrational frequencies are v(1)=923.2, v(2)=318.3, and v(3)=3829.8 cm(-1) for LiOH and v(1)=912.9, v(2)=245.8, and v(3)=2824.2 cm(-1) for LiOD. The molecule is extremely nonrigid with respect to angular deformation; the calculated deviation from linearity for the vibrationally averaged structure is 19.0degrees in the (000) state and 41.9degrees in the (03(3)0) state. The calculation not only predicts, in agreement with previous work [P. R. Bunker, P. Jensen, A. Karpfen, and H. Lischka, J. Mol. Spectrosc. 135, 89 (1989)], a change from a linear to a bent minimum energy configuration at elongated Li-O distances, but also a similar change from linear to bent at elongated O-H distances. (C) 2004 American Institute of Physics.