Effects of high magnetic fields on the properties of the lowest triplet state of the OLED emitter Pt(4,6-dFppy)(acac) are studied by site-selective high-resolution optical spectroscopy. Application of high fields strongly alters the pattern and magnitude of splitting of the triplet term, the oscillator strengths of the purely electronic 0-0 transitions between the T-1 substates and the singlet ground state, and the vibrational satellite structures. In particular, the mechanism of radiative vibrational deactivation of the lowest triplet substate I changes with increasing field from a vibronically induced (Herzberg-Teller) to a Franck-Condon induced mechanism. (C) 2009 Elsevier B. V. All rights reserved.