Positronium formation in triphenylphosphine oxide and its lanthanide and hydrogen peroxide complexes have been characterized. The low probability of positronium formation observed has been attributed to a mechanism involving competition between charge transfer at large distances induced by the positron-molecular electron interaction and positronium formation during positron molecule scattering. The molecular site responsible for positronium formation has been identified. (c) 2008 Elsevier B.V. All rights reserved.