Calcium oxide can be an effective sorbent for separating CO2 at high temperatures. The carbonation reaction is the basis for several proposed high temperature CO2 capture systems when coupled with a calcination step to produce a pure CO2 stream. Fresh calcined lime is known to be able to carbonate very readily at appropriate temperatures, but the average sorbent particle in a capture system using CaO as regenerable sorbent has to undergo many carbonation/calcination cycles. This work investigates the carbonation reaction rates in highly cycled sorbent particles of CaO (20-100 s of carbonation/calcination cycles). A basic reaction model (homogeneous model) has been proved to be sufficient for interpreting the reactivity data obtained under different conditions: partial pressure of CO2, particle sizes and other relevant operation variables for the carbonation/calcination loop. The intrinsic rate parameter was found to be between 3.2 and 8.9 x 10(-10) m(4)/mol s in agreement with other values found in the literature. (C) 2007 Elsevier B.V. All rights reserved.