Despite the vast available literature on the synergistic action between zeolites and intumescent formulations, the influence of the acidity of the zeolite on the flame-retardant properties of the materials has not yet been properly addressed. This work investigates the effect of the concentration and the strength of the acidic sites of faujasite Y zeolites on their synergistic action with an intumescent formulation composed of ammonium polyphosphate (APP) and pentaerythritol (PER) in a polypropylene matrix. The results from the limiting oxygen index, cone calorimetry and glow-wire indicate that the zeolites with higher concentration of moderate strength acidic sites can catalyse more efficiently the reaction between APP and PER, which produces phosphate esters, precursors of char, enhancing the flame-retardant properties. However, an over increase in the acidic sites strength shows the opposite effect, as the zeolite can be prematurely deactivated during the initial steps of char formation. Therefore, the increase in the concentration of the faujasite Y acidic sites with moderate strength might be a good strategy in order to obtain materials with better flame-retardant properties.