Holes in cation exchange membranes of alkali chloride cells can have a number of different causes. They do not represent a source of great danger during operation of a membrane electrolyser as long as their number is limited or their combined diameter does not exceed a certain size. Of the different methods for detection of holes in membranes, two procedures are described in detail : the cell voltage test and the leak test with measurement of the gas flow through the holes. The cell voltage test is performed with low time consumption during start-up of an electrolyser. It mainly shows the large membrane holes in a cell. In contrast to this, with the leak test all holes in a membrane are detected. However, the procedure is time consuming and is possible only with a totally drained electrolyser. It is recommended to perform these tests immediately after installing new membranes and then from time to time in order to avoid damage to the cells or to the plant chlorine system.