The fluoride content of enamel is usually expressed in ppm F- in an enamel layer with a given thickness. A common procedure is to etch away a thin layer of enamel, to measure the F content by solid state electrode and to calculate the enamel layer thickness from the amount of calcium in the etch solution. The layer thickness influences therefore directly the fluoride profiles (F- content vs distance) in enamel.In this investigation, buccal surface areas of permanent human teeth were exposed to 0.1 M perchloric acid for different periods. The resulting distances between the control and acid etched surfaces were determined by both chemical depth determination (dCa) and by an optical depth determination (dOP). These techniques were applied to the same samples. The results show in most cases a statistical significant difference between the two values of interest dCa and d0P; d0P is greater than dCa. Increased exposure times result in larger differences dOP and dCa. For long exposure times d0P is considerably greater than dCa; the difference can be as much as 50%. The reason for discrepancy between optical and chemical depth determination is most likely due to the prism structure of the enamel. SEM pictures on 0.1 M HClO4 etched specimen strongly indicate an increasing roughness of the surface with increasing etching periods.