Visible emission from C2-(B2Σu+) anions has been identified underlying the much stronger Swan band emission from neutral C2(d3Πg) radicals (henceforth C2-* and C2*, respectively) in MW-activated C/H/(Ar) plasmas operating under conditions appropriate for the chemical vapor deposition (CVD) of diamond. Spatially resolved measurements of the C2-* and C2* emissions as functions of the C/H/(Ar) ratio in the input gas mixture, the total pressure, and the applied MW power, together with complementary 2-D(r, z) plasma modeling, identifies dissociative electron attachment (DEA) to C2H radicals in the hot plasma as the dominant source of the observed C2-* emission. Modeling not only indicates substantially higher concentrations of C2H- anions (from analogous DEA to C2H2) in the near-substrate region but also suggests that the anion number densities will typically be 3-4 orders of magnitude lower than those of the electrons and partner cations, i.e., mainly C2H2+ and C2H3+. The identification of negatively charged carbon-containing species in diamond CVD plasmas offers a possible rationale for previous reports that nucleation densities and growth rates can be enhanced by applying a positive bias to the substrate.