Hydrogen bubbles evolved cathodically under conditions encountered in electrochemical machining have been studied by stop‐motion photography. Constant current densities up to 150 A/cm2 and flow rates up to 2500 cm/sec have been employed with an experimental flow channel of 0.5 mm gap width. The observed bubble size decreased strongly with increasing flow rate and increased with increasing current density. At flow rates above 800 cm/sec, the bubble size was always below 20µ, the smallest diameter resolved by the optical arrangement used. Less gas was evolved in nitrate than in chloride electrolytes under otherwise identical conditions. The hydrogen bubbles were usually confirmed to a region near the cathode. Voltage oscillations and electric breakdown coincided with the appearance of a new type of bubble.