The surface carbonation process of calcium hydroxide crystals in samples of hardened Portland cement paste has been investigated at the nanometer scale with the aid of an Atomic Force Microscope (AFM). The AFM, encapsulated in a glove-box, was operated in contact mode at ambient temperature. Successive real-time measurements were performed in (i) a pure non-reacting N-2 atmosphere, (ii) a N-2 + H2O, (iii) a N-2 + CO2 and finally (iv) in a N-2 + CO2 + H2O atmosphere, respectively. In the N-2 + H2O atmosphere, until 30-40% relative humidity, a minor change as surface smoothing with an occurring instability was detected. In the N-2 + CO2 atmosphere, no change was detected, except for some very small grains becoming after two days even a little bit smaller. However, in the N-2 + CO2 + H2O atmosphere, i. e., in a simultaneous CO2 + H2O environment, with a stepwise increase of the relative humidity until 26%-30%, on the surface of the calcium hydroxide crystals several small scattered spots were found. These spots were weakly linked to the surface and could be pushed away with the scanning tip. Under constant conditions (temperature, humidity-and CO2-content), the small spots start to grow, and after a long-term exposition of the crystals to the ambient humid air, develop a specific spherular structure. The latter is interpreted to be calcium carbonate, a result of the surface carbonation process of the initial calcium hydroxide crystals. (C) 2003 Kluwer Academic Publishers.