The carbonation of CaO-based materials at high temperatures (e.g. >600°C) is a promising method of capturing CO2 emitted from, e.g. the combustion of carbonaceous fuels. The resulting CaCO3 can be regenerated by calcination at a temperature at which the equilibrium partial pressure exceeds that of the local partial pressure of CO2 (e.g. 950°C). A process involving repeated cycles of carbonation and calcination of a calcareous material is called calcium looping. The capacity of a CaO-based sorbent to accept and reject CO2 over many cycles is governed by a number of factors, such as chemical composition, surface morphology and pore structure, all of which often evolve with cycling. The present paper investigates the underlying mechanisms controlling the evolution of the micro-structures of a series of synthetic sorbents consisting of CaO mixed with the inert supports Ca12Al14O33 and MgO. These sorbents were subjected to cycles of calcination and carbonation and were characterised using a variety of in situ and ex situ techniques. It was found that the balance between the degree of surface cracking during calcination and the extent of sintering during carbonation was responsible for changes in uptake during cycling, giving an increase in uptake for the supported CaO and a decrease for the unsupported CaO.