We studied the microstructural stability and high-temperature mechanical properties of gamma/gamma' Co-Ti-Mobased alloys of varying compositions (Co-12Ti-4Mo, Co-12Ti-4Mo-2Al, Co-12Ti-4Mo-4Al, Co-10Ti-4Mo2Al, and Co-8Ti-4Mo-4Al (at.%)) using electron microscopy and density functional theory. The Co-12Ti4Mo alloy underwent discontinuous precipitation into FCC (gamma) and D0(19) (chi) phases after aging at 800 degrees C for 500 h while the Co-12Ti-4Mo-4Al alloy maintained the gamma/gamma' microstructure. Al additions improved the stability of the gamma/gamma' microstructure by forming L2(1) (beta') precipitates along the grain boundaries, reducing the supersaturation of the gamma matrix and the FCC/L1(2) interface energy. The Co-1(2)Ti-4Mo-4Al alloy showed lower yield strength compared to the Al-free alloy, however, the difference in yield strength between these two alloys decreased with increasing temperature. This decrease is counterbalanced by a substantially enhanced specific yield strength at elevated temperatures as compared to other well-known Co-based alloys such as MarM302 and Co-9Al-9W. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.