This paper describes a. systematic modelling technique which makes it possible to avoid many of the tedious elements that always are involved in the modelling of industrial processes. The technique is based on the non-linear equations which are usually obtained from basic physical laws. It gives a systematic procedure to compute steady-stato solutions and linearized equations from the non-linear physical equations. The procedure is based on well-known methods for solving non-linear equations and numerical differentiation. The final result is a system description in state space form S(A, B, C, D). The essential difficulty is to find the smallest number of state variables and to assign these to the linearized model. A method for solving this problem is the main result of the paper. The paper also contains an application of this method to the modelling of a boiler.