This paper presents the findings obtained from an investigation into the degradation mechanisms of cementitious materials exposed to concentrated NaOH solutions. By monitoring the length change of mortar prisms, it was found that dimension instability can result from exposure to concentrated NaOH solutions. Considerable expansion was observed with portland cement mortars. This appears to be the result of precipitation of coarse calcium hydroxide and Na-rich calcium silicate gel in voids and particularly at the aggregate/paste interface. The NaOH attack on blended cements was found to be different to that on portland cement. The implications of the findings in terms of designing concrete for better resistance to NaOH attack are given.