Dynamical coupling effects between the (200) reflection and weak odd-index reflections in the Bragg position in convergent-beam diffraction patterns has been used to detect antiphase boundaries (APBs) in GaAs grown on (001) Si. It was shown that APBs lie in most cases on energetically favorable {110} planes and effectively stop the propagation of dislocations, twins and stacking faults. The formation of APBs is promoted by Si surface contamination and irregularities.