Liquid feedstocks of ethylene crackers are complex hydrocarbon mixtures containing a huge number of different chemical species. Accurate characterization of petrochemical feeds plays a crucial role in describing the different reaction pathways and reactivity of the various isomer species. Thus, this knowledge becomes a need when it comes to develop comprehensive kinetic models of steam cracking process able to select the optimal design and operation of pyrolysis coils. For this reason, several efforts are recently spent to improve feedstock characterization and analytical separation techniques. The high potential of GC×GC combined with MS detection is a clear example of these efforts. Despite recent advancements of analytical techniques, these methods give only a partial molecular reconstruction of the feedstocks and it is a need to rely also on different bases. The Shannon’s entropy criterion correctly describes the boiling curves and molecular weight distributions of different hydrocarbon classes, but this is only a first step and not the very critical one. As a matter of facts, more than the molecular weight distribution and the proper distillation curve, the isomer distribution is proved to have a critical role. Isomers with almost identical physical properties, such as the isomers of branched alkanes and cyclo-alkanes, can exhibit very different cracking behaviors, and hence, a poor estimate of their internal distribution can significantly affect the ethylene yields. Thus, the horizontal lumping becomes more critical than the vertical one, and isomer distribution needs to be characterized in a different way. This paper aims at emphasizing the role of the probability of methylation and alkylation inside the families of homologous hydrocarbons. For the first time, it is highlighted the importance of the internal distribution in the molecular feedstock reconstruction. On these bases and for these purposes, SPYRO® kinetic model is revised and extended to more than 500 species, covering a large detail of feed components, and it is now able to suitably show and quantify the effect of isomer distribution.