It has been assumed that a membrane is needed in an MEC to avoid hydrogen losses due to bacterial consumption of the product gas. However, high cathodic hydrogen recoveries (78 ( 1% to 96 ( 1%) were achieved in an MEC despite the absence of a membrane between the electrodes (applied voltages of 0.3 < Eap < 0.8 V; 7.5 mS/cm solutionconductivity).