This paper investigates the decomposition kinetics of citric acid, sodium bicarbonate and their combinations in masterbatches with low density polyethylene in view of their use in injection molding with simultaneous polymer foaming. The thermal decomposition was studied by isothermal and non-isothermal calorimetry to separate the various chemical events. A multi-step autocatalytic model was used to determine the kinetic parameters. Results show that for sodium bicarbonate compounds, the kinetics can be modeled by first-order equations while for citric acid compounds it shows two steps of decomposition due to reaction intermediates. The parameters obtained for the mono-component masterbatches can be used to model the kinetics of compounds containing a mix of sodium bicarbonate and citric acid with the same multi-step coupled equations. Experiments and modelisation show that the reaction in the masterbatch proceeds in well separated steps corresponding to each species. In particular, at high heating rates such as those encountered during melting in the injection molding machine, the citric acid decomposition occurs in a single step making the analysis of the kinetics and the modelling of the overall foaming process more simple.