The effect of carbonation on the microstructure and flexural strength of hardened aluminous cement pastes has been investigated. It is found that pastes containing either C(3)AH(6)* or CAH(10) as the major crystalline hydrate phase are readily partially carbonated at a humidity of 72.6%. The carbonation products are calcite, vaterite, which is present as 50 nm crystallites, and hydrous alumina, which is always produced in the form of an amorphous gel. Partial carbonation produces a modest increase in the flexural strength for pastes containing either C(3)AH(6) or CAH(10).