Core (r/a 0.5) toroidal rotation from argon (Ar17+, 40 AMU) and molybdenum (Mo32+, 96 AMU) ions has been compared in C-Mod tokamak plasmas over a wide range of operating conditions and confinement schemes, including Ohmic L-mode in the linear and saturated regimes, ion cyclotron range of frequencies heated I-mode and H-mode, as well as in discharges with induced locked modes and with external current and rotation drive. In all cases the velocities of the two impurities are identical within about 5%, for a range between −60 and +80 km s−1. This is in general agreement with the predictions of neo-classical theory.