A new model for DX centres is proposed and shown to be the only model which agrees with DLTS, metastability and ballistic phonon scattering experiments. In this model an As antisite complex, formed from a substitutional donor by the exchange of two adjacent atoms, binds two electrons. Furthermore, if these electrons are assumed to occupy a triplet spin state, the model can explain the paramagnetic susceptibility reported for filled DX centres. The stability of such a configuration could be due to the unusual properties of the antisite complex. Extension of this analysis to the related EL2 defect in GaAs suggests that the normal EL2 state may be 3T2(A2) and the metastability of the excited state due, in part, to spin conservation.