A method for estimation of residence time in continuous flow systems with varying dynamics is presented. By resampling, i.e. choosing time instants different from the given sampling instants, and interpolation between measured data points, we obtain a continuous flow system with constant residence time expressed in the new resampled time vector. We assume the flow patterns in the systems are invariant. The new data set is then used for identification of parameters in a chosen model structure. From the identified model, the residence time is readily calculated and a procedure for that is briefly described. The presented method is readily extended to enable use in recursive identification, in that case, however, as an improvement in the tracking ability of an ordinary recursive routine.