The bulk glass-forming alloys Zr59Ti3Cu20Al10Ni8, Zr57Cu15.4Al10Ni12.6Nb5 and Zr35Cu30Al10Ni3 were fundamentally analyzed regarding their anodic reactivity in 250 g/L NaNO3 solution - a standard electrolyte for electrochemical (micro)machining. All alloys exhibited stable passivity in a wide potential range. At potentials >2 V vs. SCE (saturated calomel electrode) a sharp breakdown of the passive state and active dissolution occur. By controlled electrolyte flow the transpassive reaction rate can be enhanced. Re-passivation from the transpassive state is not spontaneous - it requires a wider potential or time window. The influence of alloy composition and structural state will be discussed as well as consequences for the machining process. (C) 2014 The Electrochemical Society. All rights reserved.