A humidifier for use with a breathable gas supply apparatus includes a hollow body adapted for partial filling with water to a predetermined maximum water level, a gas inlet to the body above the maximum water level and a gas outlet from the body above the maximum water level. The humidifier further includes a temperature heating element for heating the water and/or an adjustable flow divider adapted to divide the interior of the body above the maximum water level into a relatively dry gas region and a relatively wet gas region. The position of the divider is variable so as to vary the relative proportion of the gas flowing from the inlet to the outlet that passes through the relatively dry and relatively wet gas regions to thereby vary the amount of humidification thereof.