(57) The invention is concerned with a method of preventing undesirable gas generation between a pair of opposite, electrically energized negative and posi­tive electrodes spaced from one another by a gap filled with an aqueous electrolyte solution. According to the invention, the positive electrode is coated with an olefinic substance to form micro-droplets thereof on the surface of the positive electrode prior to electrically energizing the electrodes such that upon electrical energization gas generated as a result of electrolysis is consumed by reaction with the ole­finic substance, the reaction being carried out in the presence of a metallic oxide catalyst. In this manner, undesirable gas generation between the elec­trodes is prevented. The method of the invention is particularly useful in electrocoagulation printing systems where an image is reproduced by electrocoagu­lation of an electrolytically coagulable colloid on a positive electrode to form dots of coagulated colloid representative of a desired image, the invention ena­bling the electrical resistance which is created at the interface of the negative electrode by the accumu­lation of hydrogen and causes an erratic formation of the dots of coagulated colloid to be suppressed.