Abstract The self-assembling conditions of identical 4B10Sb and 1B4Sb clusters were studied in GaP:(B, Sb). The conditions are considered in the ultra dilute impurity concentration ranges and from 0°C to 800°C. Co-doping with boron and Sb transforms GaP into GaP-rich BxGa1−xSbyP1−y quaternary alloy of four binary compounds BSb, BP, GaSb and GaP. The formation of clusters decreases a sum of the enthalpies of the constituent compounds and strain energy. 4B10Sb clusters self-assemble mainly and almost maximal quantities of such clusters should form up to 600°C. The insignificant density of 1B4Sb clusters should also occur in GaP:(B, Sb). The results demonstrate that GaP:(B, Sb) is a semiconductor with array of ∼1nm identical low band gap objects embedded in the wide band gap matrix.