A broadband upconversion luminescence was observed from Mo-doped titania under a 978 nm diode laser excitation. The luminescence has been assigned to the transitions from the excited states T-3(1), T-3(2) to the ground state (1)A(1) of the [MoO4](2-) radicle. The intermediate states available for upconversion are supposed to be related with Mo4+ and/or Mo5+ species. The temporal evolution of the upconversion luminescence and the excitation-power dependence of the steady-state emission intensity exhibit the typical fingerprint of a photon avalanche. The dynamical process is well described using the fluorescence 'transfer function' model. (C) 2002 Elsevier Science B.V. All rights reserved.