This study evaluated the impact of ammonia on mesophilic anaerobic digestion (AD) with thermal hydrolysis pretreatment (THP) treating a mixture of primary sludge and waste activated sludge and operated under constant organic loading rate of 9kgCOD/m3 /d. Free ammonia concentrations in the digesters were varied between 37 and 966mgNH3 -N/L, while maintaining all other operational conditions constant. A decrease in volatile solids reduction from 54±5% (at <554mgNH3 -N/L) to 35±6% at the maximum free ammonia concentration of 966mgNH3 -N/L was observed at steady-state conditions. No impact of free ammonia on final dewaterability was detected. Free ammonia thus mostly limited methanogenesis. A free ammonia Monod inhibition constant of 847±222mgNH3 -N/L for methanogens was estimated based on the digester steady-state methane rates dynamics. This study showed that current THP AD digesters (typically 110-260mgNH3 -N/L) operate under 12%-18% ammonia inhibition for methanogenesis. Operation under SRT of 15days, about 2 times more than needed to retain methanogens, can compensate for lower methanogens rates and avoid performance impacts. The later shows a good potential to operate under higher free and total ammonia concentration without jeopardizing performance. PRACTITIONER POINTS: Only from a free ammonia concentration above 554mgNH3 -N/L, decreased volatile solids reduction and biogas yield were observed. A volatile solids reduction of 35±6% at maximum free ammonia concentration of 966mgNH3 -N/L was still achieved. A Monod inhibition constant for methanogens of 847±222mgNH3 -N/L was estimated. It was estimated that current THP AD systems (110-260mgNH3 -N/L) operate under 12%-18% NH3 inhibition for methanogenesis.