ESR spectra of free lithium clusters Li-11 and Li-19 were measured by a molecular beam method. The spectra show that the observed clusters are Li-10 + Li and Li-18 + Li, i.e., Li-10 and Li-18 with an atomic Li attached to them. The host cluster Li-18 in Li-19 is highly spin-polarized by the attached atom, and gives the spectrum a large number of equally spaced hyperfine-lines indicating 18 equivalent nuclei, which reveal fluid-like behavior of Li-18 at an estimated internal temperature of T= 102 K. The host cluster Li-10 in Li-11 I is only weakly spin-polarized probably because Lilo is an ordinary solid-like cluster. (c) 2007 Elsevier B.V. All rights reserved.