A new method for feedstock reconstruction of industrially important hydrocarbon fractions is developed using as input the analytically determined commercial indices such as the average molecular weight, the specific density, the H/C-ratio, the paraffin, isoparaffin, olefins, naphthenes, aromatics (PIONA) weight fractions and a set of ASTM boiling points. The method is based on Shannon's entropy criterion and allows generating a molecular composition of a naphtha fraction that meets all the boundary conditions set by the industrially available commercial indices. The most important components present in a naphtha fraction are selected as possible feedstock components. This approach is very fast because the optimization function can be transformed from a nonlinear equation in the N mole fractions (N similar to 10(2)) into a nonlinear equation in maximally 14 parameters, making it very attractive for the petrochemical industry. A good correspondence exists between the simulated weight fractions and the actual analytically determined ones provided that the specified commercial indices are sufficiently accurate. The feedstock module is implemented in a simulation package for steam cracking but can be used as a stand-alone tool too. The combination of these two simulation tools makes it possible to obtain a good agreement between the simulated and experimentally determined product yields for a set of 50 pilot plant experiments with different naphtha feedstocks under widely varying operating conditions. (c) 2006 Elsevier Ltd. All rights reserved.