We investigated the effect of CO2 on layered Li1+z Ni1−x−y Co x M y O2 (M=Al, Mn) cathode materials for lithium ion batteries which were prepared by solid-state reactions. Li1+z Ni(1−x)/2Co x Mn(1−x)/2O2 (Ni/Mn mole ratio=1) singularly exhibited high storage stability. On the other hand, Li1+z Ni0.80Co0.15Al0.05O2 samples were very unstable due to CO2 absorption. XPS and XRD measurements showed the reduction of Ni3+ to Ni2+ and the formation of Li2CO3 for Li1+z Ni0.80Co0.15Al0.05O2 samples after CO2 exposure. SEM images also indicated that the surfaces of CO2-treated samples were covered with passivation films, which may contain Li2CO3. The relationship between CO2-exposure time and CO3 2− content suggests that there are two steps in the carbonation reactions; the first step occurs with the excess Li components, Li2O for example, and the second with LiNi0.80Co0.15Al0.05O2 itself. It is well consistent with the fact that the discharge capacity was not decreased and the capacity retention was improved until the excess lithium is consumed and then fast deterioration occurred.