The present work is based on the Van Krevelen's approach proposed by Lyon et al. [1] to determine the relationships between the molecular structure and the flammability of molecules using Microscale Combustion Calorimeter (MCC). Via this approach, the contributions to Heat Release Capacity (HRC) and Total Heat Release (THR) were successfully calculated for two phosphorus-containing groups: phosphonate and dioxaphosphorinane. We showed that the Van Krevelen's approach was invalid when the molecules contain both phosphorus and ester or acid groups. We hypothesized that these functions interact between them during the thermal decomposition. In this study we propose to calculate "Interaction Indexes" to estimate the level of interactions according to the chemical structure. The results enable to draw first conclusions about the best chemical architecture for lowering flammability.