This paper is the continuation of two articles which appeared in this journal, and presents an approach to the thermodynamics of general systems via information theory. It is composed of two parts. In the first part one proposes a modification of the initial Shannon information theory which takes into account the point of view of the receiver. The concepts of ‘ effective entropy ’ and ‘ effective information ’ are given, the relation between effective information and Shannon information is exhibited. We then obtain the concept of negative information which may explain the practical phenomenon of information loss. In the second part of the paper, the interpretation of the results of the first part provides a model of state equation which would apply in the thermodynamics of general systems. The counterpart of perfect gas, the perfect system, is investigated via evolution principles and it is emphasized that the results so obtained agree with some practical phenomena. Some suggestions for further research are given in the conclusion.