Unusually intense vibronic sidebands appear in the 4,2 K absorption spectra corresponding to transitions to sextet states F-6(J) (J = 1/2-9/2) of Sm3+ in K5Li2SmF10 single crystal. R value defined as the ratio of integrated intensity of vibronic sideband and the integrated intensity of no-phonon lines has been estimated to be as high as 2.64 for the H-6(5/2)-F-6(7/2) transition. Contribution of vibronics to transitions to quartet states is negligible. Instead, the electron-phonon coupling induces strong thermal line broadening as evidenced by temperature dependence of line widths associated with the (4)G(5/2)-H-6(5/2) transition, consistent with the electron-phonon coupling parameter alpha similar to 500 cm(-1). (C) 2003 Elsevier B.V. All rights reserved.