The demand for standardized, comparable, and transparent methods to assess product-related environmental impacts—particularly the Product Carbon Footprint (PCF)—is increasing across industry, policy, and science. The mass balance (MB) is another assessment tool used primarily in the chemical industry to attribute sustainable feedstock to products. This paper examines the methodological and practical challenges as well as the potential of integrating MB in life cycle assessment (LCA) according to ISO 14040/44/67. While the MB method allows the book-keeping-based attribution of sustainable raw materials to products—without the need for physical segregation—LCA relies on actual material flows and correlating environmental impacts. This leads to differences in system logic and goals, making integration challenging. Using a fictional example from the chemical industry, several scenarios are defined and discussed to explore the methodological compatibility of both approaches (MB and LCA). The paper demonstrates that credible integration is particularly feasible when physical connectivity, functional equivalence, and site-specific system boundaries are respected. At the same time, there is further an urgent need for global standardization to ensure methodological consistency and verifiability, for example, with the help of certification frameworks. The findings contribute to the scientific advancement of combined sustainability assessment methods and the operationalization of PCF certification schemes that incorporate MB principles.