The confined etchant layer technique has been applied to achieve effective three-dimensional (3D) micromachining on n-GaAs and p-Si. This technique operates via an indirect electrochemical process and is a maskless, low-cost technique for microfabrication of arbitrary 3D structures in a single step. Br-2 was electrogenerated at the mold surface and used as an efficient etchant for n-GaAs and p-Si; l-cystine was used as a scavenger, for both substrates. The resolution of the fabricated microstructure depended strongly on the composition of the electrolyte, and especially on the concentration ratio of l-cystine to Br-. A well-defined, polished Pt microcylindrical electrode was employed to examine the deviation of the size of the etched spots from the real diameter of the microelectrode. The thickness of the confined etchant layer can be estimated, and thus the composition of the electrolyte can be optimized for better etching precision. The etched patterns were approximately negative copies of the mold, and the precision of duplication could reach the micrometer level for p-Si and the submicrometer level for n-GaAs. Although the same etchant (Br-2) and scavenger (l-cystine) were used in the etching solutions for GaAs and Si, the etching process, or mechanism, is completely different in the two cases. Compared with the fast etching process on GaAs in an etching solution with a concentration ratio of 3:1 of l-cystine to Br-, the concentration ratio needs to be 50:1 for etching of Si. For the micromachining of Si, the addition of a cationic surfactant (cetyltrimethylammonium chloride, CTACl) is necessary to reduce the surface tension of the substrate and hence reduce the influence of evolution of the byproduct H-2. The function of the surfactant CTACl in comparison with an anionic surfactant (sodium dodecyl sulfate) was studied in contact-angle experiments and micromachining experiments and then is discussed in detail.