Absorption and luminescence measurements of Tm3+ and Ho3+ ions in LiYF4:Tm3+, LiYF4:Ho3+ and LiYF4:Tm3+ Ho3+ crystals were carried out. The data collected were used to determine the Ho3+ absorption coefficient integral for I-5(8)-->I-5(7) transition, and the overlap integral between the normalized Tm3+ luminescence spectrum due to the F-3(4)-->H-3(6) transition and the Ho3+ absorption band due to I-5(8)-->I-5(7) transition. The relevant critical transfer distance (R-o), that gives a measure of the Tm3+-Ho3+ coupling, was determined considering the Forster type interaction between the ions. It was found to be 22.5 Angstrom and 28.8 Angstrom at 300 and 78 K, respectively. (C) 2000 Published by Elsevier Science B.V. All rights reserved.