Using the density operator formalism, an analytical model is developed to study the time evolution of the electron spin magnetization of photo-excited triplet states in an external magnetic field. Analysis reveals that pulsed light excitation initiates an oscillatory electron spin magnetization in the direction of the external field. The frequency of the quantum oscillations is determined by the electron Zeeman and dipolar interactions. The oscillation amplitude reaches a maximum when the electron Zeeman splitting matches the energy of a zero-field transition of the triplet state. This suggests that the predicted quantum oscillations can be detected only at low magnetic fields. (c) 2007 Elsevier B.V. All rights reserved.