Recently the effect of hydroxylamine (NH2OH) on the etching characteristics of alkaline solution (e.g. potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH)) is studied to obtain improved etching characteristics, especially high etch rate and enhanced undercutting at convex corners. Alkaline solution modified with NH2OH provides improved etch rate of silicon (Si), high etch selectivity between silicon and silicon dioxide (SiO2), considerably high undercutting at convex corner. As the addition of NH2OH alters the etching properties of alkaline solution dramatically, it is indispensable to investigate the effect of aging of NH2OH-added alkaline solution on its etching characteristics. In this work, the influence of aging of 15% NH2OH-added 20 wt% KOH, which is referred to as an etchant, on the etching characteristics of Si{100} and Si{110} is studied in detail. The results are systematically presented. The etch rate of silicon and the undercutting at convex corners decrease significantly with etchant aging at etching temperature, while the etch rate of silicon dioxide and etched surface morphology are not considerably affected with the age of the etchant. This study is significantly important for the researchers and industries where silicon wet anisotropic etching is used for silicon micromachining. © 2019 The Electrochemical Society.