The effect of conversion treatment parameters on the characteristics of conversion coatings was studied in order to determine the optimal elaboration conditions, using statistical experimental designs. These conversion coatings must be characterised by strong interfacial adhesion, high roughness and high real surface area, which were measured by an electrochemical method. The influence of all the elaboration factors (temperature, time and bath composition: sulphuric acid, thiosulphate (accelerator), propargyl alcohol (inhibitor)) and also the interactions between these factors were evaluated, using 2(5-1) fractional factorial design. A polynomial model was deducted from statistical analysis of the experiments. The evaluation showed, by using a designed experimental procedure, that the most important factors were sulphuric acid, thiosulphate and alcohol concentrations. But the growth of the conversion coating is also influenced by the interaction effects between the bath compounds. With such optimised conditions, the real surface area of conversion coating obtained was about 200 m(2) m(-2). This conversion coating modified by alumina deposit increases the thermal oxidation resistance of the Fe-C alloy. A discussion of the use of experimental designs for increased understanding and optimisation of conversion coating process is also given.