A rapid and accurate method is presented for measuring the transfer function of seismic channels by using the step-excitation technique. The parameters of the Laplace polynomials are estimated by inverting the step response of the channel. Special attention is paid to the estimation of reliable confidence bands: the statistical structure of the observed residuals is taken into account. An example is given for S-5007 long-period instruments coupled to four-order bandpass filters. Accuracies of one per cent and one degree for the amplitude and phase response respectively are obtained in the passband by stacking a great number of identical pulses. Advantages of the method are analytical transfer function description, high accuracy and dependable error estimation.