In a number of nickel-based superalloy systems strengthened by gamma prime (γ′), eta-phase (Ni3Ti, η) forms during prolonged high-temperature exposure, but its effect on mechanical properties is not well characterized. Using thermodynamic modeling and design-of-experiments techniques, three modifications of the nickel-based superalloy Nimonic (Nimonic® is a trademark of Special Metals Corporation group of companies.) 263 were identified that yield increased volume fractions of the eta-phase (Ni3Ti, η) at temperatures above 1023 K (750 °C). Volume fractions of η-phase were evaluated for each alloy and heat-treatment condition using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Compared to Nimonic 263, small additions of V and Ta were found to increase the volume fraction of η-phase above 1023 K (750 °C) from approximately 5 pct to above 15 pct, thus providing a route for future mechanical behavior experimental studies, which was not in the scope of this work.