Wet chemical etchants based on ammonium hydroxide-water (AHW) solutions for micromachining monocrystalline silicon are described. The etchants can be applied under clean-room conditions within standard IC fabrication lines. The nature of AHW solutions is discussed with respect to anisotropy and selectivity. At 75 degrees C and 9 wt. per cent AHW, a maximum etch rate for (100) silicon of approximately 30 mym/h has been determined. (111):(100) etch rate ratios show values around 4 per cent. AHW etchants exibit an excellent selectivity to SiO sub 2, Si sub 3 N sub 4 and highly boron-doped silicon. Aluminium will not be attacked by silicon-doped AHW etchants. Therefore, standard IC metallization techniques can be used for device fabrication. The application of AHW solutions to the fabrication of basic micromechanical structures is demonstrated.