A study is made of the mechanisms of the commercially important phase transformation of Al-6(Fe,Mn) to alpha-Al-(Fe,Mn)-Si phase during homogenisation of direct-chill (DC) cast ingots of AA3104 alloy. Annealing a diffusion couple of an Al-Fe-Mn alloy (containing only Al-6(Fe,Mn) particles) and an Al-Si alloy permits study of the effects of steadily increasing silicon content, and thereby of the nucleation stage of the transformation. Initially the Al6(Fe,Mn) particles undergo the eutectoid transformation (to alpha-phase and ccp Al) typical for DC-cast alloy, the highly deformed microstructure of the roll-bonded diffusion couple facilitating nucleation of the transformation. Later, the aluminium matrix recrystallises, and nucleation is limited to selected particles on the grain boundaries; in that case the majority of Al-6(Fe,Mn) particles do not transform by the eutectoid mechanism. Markedly coarser alpha-phase particles indicate that the Al-6(Fe,Mn) particles instead transform by dissolving and supplying transition metals for growth of alpha-phase particles.