In this paper, we study the robustness of a group-delay-based method for determining the instants of significant excitation in speech signals. These instants correspond to the instants of glottal closure for voiced speech. The method uses the properties of the global phase characteristics of minimum phase signals. Robustness of the method against noise and distortion is due to the fact that the average phase characteristics of a signal is determined mainly by the strength of the excitation impulse. The strength of excitation is determined by the energy of the residual error signal around the instant of excitation. We propose a measure for the strength of the excitation based on Frobenius norm of the differenced signal. The robustness of the group-delay-based method is illustrated for speech under different types of degradations and for speech from different speakers. © 1999 IEEE Publisher Item Identifier S 1063-6676(99)08075-X.