Two high entropy alloy (HEA) compositions were compared to explore the individual effects of Co and Ni on phase stability and resultant deformation response in Fe-Mn-Cr-Si-containing HEAs. It was observed that Co-Si-containing HEA depicted responsive phase evolution upon friction stir processing owing to decreased γ (f.c.c.) matrix stability as against Ni-Si-containing HEA. As a result, the Co-Si HEA showed the presence of dual-phase microstructure with the dominance of ε (h.c.p.) phase (52 pct), whereas the Ni-Si HEA showed single-phase γ microstructure under similar processing condition. Also, the dominant deformation mechanisms were different in the two alloys. Co-Si HEA showed uniform strain partitioning between the f.c.c. and h.c.p. phases. Conversely, single-phase f.c.c. microstructure in Ni-Si HEA accommodated strain by twinning-induced plasticity. © 2018, The Minerals, Metals & Materials Society and ASM International.