Amorphous carbon nitride (a-CNx) films prepared by RF magnetron sputtering technique display a moderate electrochemical reactivity. However, after electrochemical activation by a cathodic pretreatment in acidic solution, they exhibit a quasimetallic behavior. This behavior has been notably assessed by scanning electrochemical microscopy (SECM). The approach curves show that the a-CNx electrode surfaces are made of active and inactive domains. These surfaces have also been characterized by atomic force microscopy (AFM) and XPS. AFM shows that the a-CNx surface roughness remains very low after the activation treatment. XPS analyses indicate that the electrochemical activation of the a-CNx materials is associated with a decrease in the number of superficial nitrogen atoms involved in sp3 CN bonds. Copyright © 2006 John Wiley & Sons, Ltd.