Apparatus for generating gas by electrolysis of a liquid comprises an electrolysis cell, and first, second and third chambers. Liquid for supply to the electrolysis cell is stored in the first chamber. The second chamber contains liquid which is a product of the electrolysis reaction and is enriched with a first gaseous product of the electrolysis reaction, and has an outlet for controlled release of the first gaseous product. The third chamber contains liquid which is a product of the electrolysis reaction and is enriched with a second gaseous product of the electrolysis reaction, and has an outlet for controlled release of the third gaseous product. A first channel which connects the first and second chambers, and a second channel which connects the second and third chambers. Each of the first and second channels provides a trap for liquid which can provide a barrier to passage of gas between the chambers which are connected by it, irrespective of the level to which liquid in the first chamber falls during operation.