The selectivity of the EDC (1,2-dichloroethane) cracking process in vinyl chloride monomer (VCM) manufacturing strongly depends on the cracking severity and the EDC feed quality. A strong decline of selectivity is observed for EDC feed purities below 99.5 wt % or cracking rates above approximately 60%. The selectivity of the cracking process also deteriorates in the course of the plant's operating time as a result of accelerated coke formation. Reduced selectivity in the process of EDC cracking results in increased marginal costs (additional costs per each additional tonne of VCM). This is especially relevant for a VCM plant running near full load. The marginal costs can be expressed as a function of the cracking rate and the resulting costs for processing the uncracked portion of EDC. With a new and simplified EDC cracking simulation model for the energy input and conversion, together with a simplified reaction model for the definition of the corresponding product spectrum, the cost impact of the aforementioned influences on the cracking process can be estimated in advance based on the current production rate and plant performance.