In this study, two strategies, thermal pretreatment and chemical doping, were investigated as a method of improving the residual carrying capacity of Longcliffe and Havelock limestone for calcium looping systems. Four parameters were varied during thermal pretreatment: temperature (900-1100 degrees C), time (3-12 hr), gas composition (0-100 % CO2 balanced in N2) and particle size (90-355 micrometre). After pre-calcination, the sorbents were subjected to 20 carbonation-calcination cycles performed in a thermographic analyser (TGA) to monitor any signs of sorbent improvement. The degradation of sorbent activity was modelled using the decay equation suggested by Grasa and Abanades (2006). Both Longcliffe and Havelock samples showed self-reactivation when pretreated under CO2, however this did not result in a greater carrying capacity after 20 carbonation/calcination cycles compared to the untreated limestone. For chemical doping, Longcliffe doped using 0.167 mol % HBr via quantitative wet impregnation method resulted in an increase in residual carrying capacity of 27.4 % after thermal pre-treatment under CO2 when compared to the untreated but doped limestone, assuming self-reactivation continued as modelled. When Longcliffe was doped and then pretreated under pure N2, the limestone showed self-reactivation, which was not seen in the undoped sorbent when also pretreated under N2. Thus, the success of pretreatment may be dependent on the chemical composition of the limestone. Finally, BET surface area and BJH pore volume analysis was used to understand the changes in the sorbents' morphologies. The closure of the mesopores (dpore<150 nm) after the pretreatment was correlated to the self-reactivation in the subsequent cycles.