Third molars just erupting and premolars (before eruption or half a year or 3.5 years after eruption) were extracted and sawn vertically through the buccolingual plane. Tracings of the Ca, P, Na and Mg contents of the outer 100-µm layer of the enamel were made with an electron microprobe. Care was taken to limit the diameter of the excited volume to about 1 µm and to suppress the evaporation of Na. On the unerupted and freshly erupted surfaces the Na and Mg content varied from 50 to 80% of the content at 100 µm depth. However, half a year or 3.5 years after eruption the Na and Mg contents were negligibly small at the tooth surface and they remained smaller than in unerupted or freshly erupted teeth up to a depth of 10–30 µm. These data suggest that posteruptive maturation occurs in the mineral of the outer layer of tooth enamel whereby it builds up a higher resistance to carious breakdown. This interpretation is corroborated by clinical findings reported in the literature. The consequences of wear and abrasion on these phenomena are not known yet. The above-mentioned depth of 10–30 µm agrees with the penetration depth of pH changes estimated to result from the consumption of foods and drinks with varying pH.