Isothermal reversible reaction of a solid mixture phase \(\upgamma\) which is transformed into two (or more) different solid (pure, binary compound or mixture) phases \(\upalpha\) and \(\upbeta\) during the cooling of a system. In a binary system, \[\ce{\upgamma <=> \upalpha + \upbeta}\] where the forward arrow indicates the direction of cooling. The equilibrium reaction occurs along the eutectoid line at the eutectoid temperature. At the eutectoid composition, the compositions of the high temperature solid phase and the mechanical mixture of the low-temperature solid phases are equal, and intermediate to the compositions of the pure solid phases \(\upalpha\) and \(\upbeta\). This composition and the eutectoid temperature, which are isobaric invariants of the system, define the eutectoid point.