For the first time red-to-blue photon avalanche upconversion has been discovered and investigated in LiKYF5 crystals doubly doped with TM3+ and Er3+ under laser excitation at the liquid nitrogen temperature. On the basis of the optical studies, an original model of the luminescent mechanisms, within which the Er3+ ions act as both donor and acceptor, and as a result sensitize the photon avalanche effect in TM3+ by converting the optical excitation energy in its diffusion among the TM3+ ions, is presented. It is assumed that all these mechanisms can be realized when the Er3+ concentration in crystals is relatively low in order to avoid direct interactions between the Er3+ ions. (C) 2002 Elsevier Science B.V. All rights reserved.