A two-step procedure for estimating linear simultaneous structural equations with unit roots is presented. It generalizes the procedure of differencing for univariate time series via G. Box and J. M. Jenkins (1970). First, one finds the number of unit roots and the canonical variables that are stationary. Second, one retains only the stationary canonical variables and estimates a stationary model by standard methods. The procedure is easy to use. It is robus t against the difficult testing problem of finding the correct number of unit roots. A multiplier-accelerator model is estimated with interesting conclusions.