Calcium oxide can be an effective CO2 sorbent at high temperatures. When coupled with a calcination step to produce pure CO2, the carbonation reaction is the basis for several high-temperature separation systems Of CO2. The formation of a product layer of CaCO3 is known to mark a sudden change in the reaction regime, from a very fast CO2 uptake to very slow carbonation rates. The critical thickness of this product layer of CaCO3 has been measured in this work on real sorbent materials, using different limestone precursors and submitting them to many repeated carbonation calcination cycles (up to 100). Mercury porosimetry curves of the calcines and their carbonated counterparts have been obtained and their differences interpreted with a simple pore model, from which the thickness of the product layer is derived. An average value of 49 nm (+/- 19% standard deviation) has been obtained, which is quite insensitive to the type of limestone and to the texture of the calcine as long as the model is fulfilled. The implications of this value on our understanding of the sorbent performance in these CO2-capture systems are discussed.