A gas burner system which can be used in a conveyor-type oven has a burner tube (4) extending across the conveyor. Gas is burnt through a slot (8) along the tube to give a ribbon flame. The tube (4) is divided into separate compartments (15-19) which connect with different lengths of the slot (8). Combustible gas is fed under independent control to the different compartments (15-19), and the slot lengths are adjustable. The slot lengths consist of short edge-region slots and multiple, longer central region slots.