A respiratory humidifier chamber for use with a breathing assistance apparatus is described. The chamber comprises a water-tight vessel having an inlet for receiving gases from the breathing assistance apparatus, and an outlet through which heated humidified gases exit the vessel for delivery to a patient. The chamber has a partition which divides the chamber into upstream and downstream sub-chambers. The sub-chambers are separated by the partition except for an aperture which passes through the partition and allows gaseous communication between the sub-chambers. The downstream sub-chamber includes a heater base which heats a volume of water contained within it The aperture is located above the volume of water. The upstream sub-chamber contains an internal heater which heats gases passing through it from the inlet to the aperture. At least part of the internal heater is located immediately adjacent the aperture.