The theory of determining the impulse response of a system by crosscorrelating the output of the system with the input is outlined. It is pointed out that pseudorandom binary input codes prove particularly convenient in facilitating interpretation of the results, and some useful properties of a class of these codes known asmsequences are enumerated.A source of error will occur in experiments using these sequences when the modulator is unable to follow the sequence exactly, and instead takes a finite time for the transition between the two binary states. The form of an error signal convenient for describing these transition delays is determined, and the cross-correlation function of the perfect sequence with this error signal is evaluated. Distinction is made between two cases, corresponding to reversible and nonreversible interstate transitions (i.e. whether or not the transition from the +1 state to the −1 state is accomplished in the same manner as that from the −1 state to the +1 state).The effect of the finite bandwidth of a perfectmsequence input on the measured response is determined by an analysis in the time domain; the results are extended to include the alteration in bandwidth due to reversible-transition errors. The method does not prove convenient for analysing the effect of bandwidth of nonreversible-transition errors to more than a first approximation of the impulse response. However, this result enables criteria to be developed to ensure that nonreversible transitions will give rise to no more serious effects on the measured response than reversible transitions.