An experimental and computational study of the polymorphism of o-nitrophenyldiphenylphosphine oxide (1) has been carried out. Two polymorphs of 1 have been obtained after a comprehensive screening procedure and their structures have been determined by single crystal diffraction. Both forms present the same racemic mixture of the Ps and Ms enantiomers, where NO2 is in a syn conformation relative to the PO group, thus resulting in the presence of a short POO2N contact (ca. 2.97 A). The anti conformation has not been found in any of these polymorphs, thus suggesting the attractive nature of the POO2N intramolecular interaction. However, MP2 computations on various model systems led to the conclusion that this interaction is energetically destabilizing. An energy analysis of all the symmetry-unique pairs found within the first-nearest neighbour of the experimental polymorphs (all with a syn conformation), and of their corresponding (non-observed) anti conformations, reveals a slightly higher stability of t