The effect of 2 at.% Ru addition on the elemental partitioning and microstructural evolution of a base Co-8.8Al-7.3W at.% superalloy, consisting of a gamma -(fcc) matrix with gamma '-(L12 structure) precipitates is studied using scanning electron microscopy and atom-probe tomography. Ruthenium partitions to the gamma '-precipitates in the Co-9.4Al-7.5W-2.1Ru at.% alloy with a partitioning coefficient, = 1.27, after aging at 900 degree C for 16 h, in contrast to the behavior observed in Ni-base superalloys and theoretically predicted for Co-base superalloys, for which Ru partitions preferentially to the gamma -phase. The addition of ruthenium does not significantly affect the gamma ' volume-fraction or the coarsening kinetics of the gamma ' precipitates compared to the base ternary alloy. The addition of Ru also leads, however, to a rapid discontinuous transformation of ( gamma plus gamma '), which initiates at the grain boundaries after 128 h aging at 900 degree C; ( gamma plus gamma ') is transformed into a lamellar phase mixture containing Co3W (D019), fcc solid-solution ( gamma ), and Co(Al,W) (B2). After 256 h aging at 900 degree C in the Ru-containing alloy, some grains have completely transformed, although regions of gamma plus gamma ' persist. The base ternary Co-Al-W alloy does not exhibit a discontinuous transformation and contains a ( gamma plus gamma ') microstructure up to 1024 h of aging at 900 degree C.