The enamel of four human impacted permanent molar teeth was used in the study. Five samples of prismatic and aprismatic enamel were made for each tooth. One sample from each group served as control, while the others were kept in the acidulated beverage (?Cola?-pH 2.57) for 15, 30, 60 and 120 minutes. Samples were then prepared for electronic microscopy (SEM) and analysed. Essential differences were noticed in the erosive effect of the phosphoric acid on the prismatic and aprismatic enamel, which is attributed to morphological differences. An irregular form of dissolution was noticed on the aprismatic surface of the enamel, expressed by extensive erosions of the rough bottom and irregular margins close to the intact enamel, even in samples with the longesVexposure to acid. On samples of prismatic enamel surfaces the first changes occurred 30 minutes after the acid exposure. The first to dissolve are the centres of prisms, followed by prism margins and in samples with 120-minute exposure even the interprismatic enamel.
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