A simple heat budget model for a radiation-dominated mixed layer of constant depth is presented. In this model the influence of the vertical irradiance (heat flux) profile is examined by means of the Jerlov <1976> optical water type classification. It is shown that the vertical irradiance profile is important in determining the mixed layer heating rate. The heating rate varies greatly as a function of water type, mixed layer depth, and diffusivity beneath the mixed layer, ranging from 0.098¿C/day for oceanic water type I with mixed layer depth of 20 m and diffusivity beneath the mixed layer of 1.0 cm2 s-1 to 0.316¿C/day for coastal type 9 with a mixed layer depth of 10 m and zero diffusivity beneath the mixed layer, a variation of more than a factor of 3.