A supercooling diagram for Al – Co alloys from 70 to 85 at.% Al is determined for cooling rates from 2 to 10 K/min. In addition, a metastable phase diagram for the rapid cooling regime (splat cooling) is constructed. With the help of these diagrams, the observed non-stable and metastable states in Al – Co alloys are discussed. Non-stable states were observed for as-cast alloys with compositions between 75.75 and nearly 77 at.% Al after annealing at temperatures from 1070 to 1110 °C, these states are composed of three phases, whereas one of these phases is liquid. The crystallization from the liquid of these samples after quenching into water follows the metastable phase diagram. The phase transformations from M-Co4Al 13 and O-Co 4 Al 13 into the high-temperature phase Co 4 Al 13 (h) during heating is described with the aid of micrographs. By quenching, the high-temperature phase cannot be stabilized at room temperature; a metastable Co 4 Al 13 (h m ) (=Y m , m = metastable) phase is retained by this procedure. At room temperature, only the phases Al, Co 2 Al 9 , O-Co 4 Al 13 , M-Co 4 Al 13 , CoAl 3 , Co 2 Al 5 and CoAl exist in equilibrium. For the construction of the equilibrium phase diagram of the Al – AlCo system, the heat-treatment approach is indispensible.