The aim of this paper is to apply fuzzy/possibilistic optimization focus in the context of the pyrolysis of ethane. The methodology is based on the fact that minimal/maximal values of these parameters are target values along the year rather than fixed real numbers. The fuzzy model is obtained taking into account the behaviour of the ethane conversion, steam/hydrocarbon ratio, inlet pressure and inlet temperature. The results demonstrated that the degree of optimism, alpha = 1, describes better the real behaviour of the process with an equilibrium approach dimensionless parameter (EA) below 0.65 and a minimum total cost of US$ 2.55. (C) 2007 Elsevier BN. All rights reserved.