Abstract: It is still an open question whether the statistical ratio of 1:3 for the formation of singlet vs. triplet excitons in organic semiconductors can be violated. In this work, we use a two-site and a multi-site model to investigate this problem. We show that spin mixing by hyperfine fields leads to a violation of the statistical ratio if singlet and triplet excitons are formed with a different rate. This only occurs if the exciton formation rate is slow enough compared to the hyperfine precession frequency. The violation of the statistical ratio can be suppressed by applying an external magnetic field. In the multi-site model we demonstrate how disorder and Coulomb interactions affect the fraction of singlet excitons formed. © 2010 Elsevier B.V. All rights reserved.