Thirty-six noncarious human molars were extracted and stored in saline. The buccal surfaces were ground wet on 400-grit silicon carbide paper to create a flat surface. The enamel and dentin surfaces were treated with 35% phosphoric acid gel, 10% phosphoric acid gel and 10% maleic acid gel for 15 and 60 seconds. Scanning electron microscope analysis showed that all the acid etchants modified the micromorphological appearance of enamel and dentin surfaces independent of the type of acid, the etching time and the concentration. On the enamel surface, the 35% and 10% phosphoric acid gels and the 10% maleic acid gel applied for 15 or 60 seconds preferentially removed the prism core material. In the specimens treated for 15 seconds with 10% maleic acid the prism core material was partially removed. On the dentin surface, the phosphoric acid gels (35% and 10%) and the 10% maleic acid gel applied for 15 and 60 seconds removed the smear layer and opened the dentinal tubule orifices; however, the dentinal surface etched for 15 or 60 seconds with 10% maleic acid gel showed residues of the smear layer.