This article develops an approximate (quasi-one-dimensional) approach to describe the combustion of liquids with a free surface. It is assumed that the distributions of characteristic quantities in the internal and external regions of the diffusion flame formed by combustion of the vapor jet are uniform. The combustion of liquids with a free surface is a complex process that consists of a number of sequential stages, in which liquid evaporates and vapor jets form above the surface, whereupon the vapor moves and reacts with the oxidizer contained in the surrounding medium. It is determined that the rate of combustion of a liquid layer of finite thickness located on a solid surface or the surface of a semiinfinite layer of inert liquid depends on the intensity of heat exchange with the phase boundary.