The bainitic transformation in Cu-Zn-Al alloys is known to have both martensitic and diffusion-controlled natures. To study the relation between these two reactions involved in the bainitic transformation, the present authors have performed a series of investigations mainly by optical and electron microscopy. A mechanism whereby directional diffusion of solute atoms stimulates the nucleation and growth of bainite is proposed based upon these investigations. The mechanism is supported by an additional experiment in which a local solute concentration in the matrix crystal is directly measured around the bainite plates using a special analytical electron microscope.