Abstract The effects of reducing the W concentration (and concomitantly mass density) of Co-10Ni-6Al-xW-6Ti at% Co-based superalloys, with a γ(f.c.c.) plus γ′(L1 2 ) microstructure, is investigated for a series of alloys with W concentrations of 6 at% (baseline alloy), 4 and 2 at% (W-reduced) and 0 at% (W-free). The γ′(L1 2 ) solvus temperature decreases strongly (by 46 °C per 1 at% reduction in W), while the liquidus and solvus temperatures decrease mildly (3–5 °C per 1 at% W reduction) as measured by differential scanning calorimetry. Scanning electron microscopy was used to image phase formation in all alloys. When aging at 900 or 1000 °C for 256 h, the W reduction does not result in the appearance of additional phases within the grains beyond the γ(f.c.c.) and γ′(L1 2 ) phases present in the original alloy. Grain-boundary precipitates are present for all alloy compositions: W-rich or W-free precipitates after aging at 1000 °C, and coarsened γ′(L1 2 ) precipitates after aging at 900 °C. The composition of grain boundary precipitates were measured with energy-dispersive X-ray spectroscopy. Vickers microhardness values decrease with decreasing W content, due to a reduction in γ′(L1 2 ) precipitation and reduced solid-solution strengthening.