Solid-solid reactions are important in many chemical and metallurgical process industries, as also in the semiconductor and electrochemical materials industries. In comparison to their fluid counterparts, these systems are difficult to model with any degree of rigour, but models derived from the fluid-solid literature have often been used with success to interpret the data. There are lacunae, however, and our previous work(1) has shown the inadequacy of the conventional models when reactions take place through a network of some complexity. In that work, the conversion of alumina and calcia to C(3)A was shown to be a series network, with C(12)A(7) being the (stable) reaction intermediate. The second reaction step of that network is studied here, namely the conversion of C(12)A(7) to C(3)A by reaction with calcia. It has been shown that this reaction takes place in a single step. While conventional wisdom treats such reactions as taking place invariably at a (moving) reaction front, it has been shown that a proper interpretation of data requires cognizance of a volume reaction component, which may, under conditions of severe diffusion limitations, be approximated well by a front reaction. (C) 2007 American Institute of Chemical Engineers.