The phase equilibria in Fe-Ni-Al and Fe-Ni-Al-Co systems have been investigated by the diffusion couple technique, and the miscibility gap that separates α1 (Fe-rich disordered bcc phase) from α2 (NiAl-rich ordered bcc phase) has been determined. It has been ascertained that the three-dimensional shape of the miscibility gap is not simple helmetlike, but has a peculiar ridge at the order-disorder transition temperature. The miscibility gap is narrowed and shifted to the Fe-rich corner by the addition of Co, and Co atoms are distributed to the α2 rather than the α1 Furthermore, by alloying Co, the fcc γ phase is stabilized and the miscibility gap between α1 and α2 is hidden to some extent by the phase regions concerned with the γ phase.