"Tracer" or Residence time distribution (RTD) studies are commonly exploited as a means of developing an understanding of the "mixing" status of vessels of various types. The effort involved in the setting up of such plant studies can be considerable and the execution of the experiment its self is often a test of endurance. In the past full interpretation of the results has not been easy and as a consequence superficial treatments have been employed. This paper presents an alternative method for interpreting RTD data, which is relatively easy to use and addresses some of the weaknesses of more conventional methods. An extention to the "tanks in series" concept is presented (ETIS) and united with the "reactor network" formulation. The suitability and appropriateness of the model is discussed and compared with the "closed" dispersion model