The neutralizing characteristics of MSWI bottom ash over a pH range between 4 and 12 were investigated using a reaction path model. Experimental data on the pH titration combined with the geochemical simulation were used to obtain the reacted fractions of the main constituents that are Ca hydroxide and carbonate and Al and Si oxides/hydroxides. The simulation results combined with the sensitivity analysis of the reaction system provided an insight into the neutralizing processes of MSWI bottom ash. The geochemical simulations showed the reactivities of major constituents of the solid waste in neutralizing reactions. In the most interesting pH range (pH4–12), the calcium‐containing constituents and carbonates made large contributions to the acid neutralizing capacity of the solid waste. The contributions of Si‐ and Al‐component to the neutralizing reactions were more complex, which could be explained by the interaction of multiple phases and equilibria in the system. Secondary phases played an important role in the neutralization reactions. Although the composition of individual secondary phases to a large extent was not known in detail, the system was robust in the sense that several possible secondary phases gave similar acid neutralization curves. Furthermore, they predicted similar aqueous phase concentrations. The predicted concentrations agree reasonably well with the experimentally found values.