Ionization efficiency curves (IECs) were measured for Li-n(OH)(n-1)(n = 2 - 5) clusters generated by reactions of laser-ablated lithium metal with water vapor, using photoionization time-of-flight mass spectrometry. The measured IECs are well-reproduced with a simulation involving Franck-Condon factors, enabling to identify the global-minimum structure of the clusters predicted in our theoretical study [Tanaka et al., J. Chem. Phys. 113, 1821 (2000)]. Hyperlithiated structures (HLS), in which an excess electron is delocalized, ate identified for the cluster with n = 2 and 3, while electron-localized or segregated structure (ELS) are for those with n = 3-5. Adiabatic ionization energies were determined as 4.053+/-0.003 eV for HLS of Li2OH, 3.687+/-0.003 eV for HLS of Li-3(OH)(2), 4.133+/-0.003 eV for ELS of Li-3(OH)(2), and 3.418+/-0.009 eV for ELS of Li-4(OH)(3). The ionization energy of Li-5(OH)(4) was determined as 3.60+/-0.11 eV by a linear extrapolation of IEC. For Li-3(OH)(2), the energetics of two lowest-energy isomers is discussed based on the observed abundance ratio.