Transient diffusion-controlled reactions in a binary suspension of equal-sized spheres are studied in a region of the equilibrium phase diagram, where a separation of time scales occurs. In Ising model language, the suspension consists of hard spheres with an internal classical spin taking two values, with a square well Ising interaction and with a magnetic field present. Transitions can occur whenever a pair of spheres approaches within the range of the Ising interaction. A separation of time scales is observed near the ferromagnetic phase transition. This allows a simplified phenomenological description of the relaxation of magnetization.