A two equation model that explains the simultaneous determination of wage and price inflation and their interaction with inflationary expectations (of the adaptive type) and real variables is presented. A dynamic definition of the long run trade off between inflation and unemployment is introduced and applied to the model in order to find conditions under which the model is stable or, in economic terminology, has a long run trade off. The model is then applied to the U.S. economy during the period 1949-1970. It is found that for all speeds of adjustment in expectations between 0 and 1 there is a permanent trade off.