In this paper, a methodology is presented for the computer generation of representative random molecules for heavy hydrocarbon mixtures which satisfy the average chemical characteristics of the mixture as determined from elemental analysis and NMR data. In the computer generation of random molecules, the molecules are represented by a connectivity matrix and a few Boolean vectors characterizing the chemical nature of each atom in the molecule. The generation of these random molecules would then make it possible to consider the reaction of a complex feed in terms of the reaction of a large number of representative molecules.