The classical theory of reaction rates deals with reactions between molecules in a homogeneous solution or gas at a constant temperature. The first reaction takes place at the slag-metal interface, the second probably at the surface of gas bubbles in the metal. Many important reactions occur heterogeneously, i.e. at the interfaces between different phases in the system; examples are the slag-metal reactions in refining processes and the reactions in the Bessemer process between the metal and the air bubbles passing up through it. Non-stationary solutions of the diffusion equations are important in innumerable problems of metallurgical reactions; e.g. heterogeneous reactions at interfaces between phases, dissolution, precipitation, etc. In a slag-metal reaction in a well-stirred bath, the composition of the liquid is kept fairly constant by the turbulent mixing and the rate-controlling process is then the diffusion through a thin interfacial zone between the slag and metal.