[en] ICRF provides Alcator C-Mod with up to 3.5 MW of power at 80 MHz using two double-strap antennas. By varying the magnetic field and the mix of ion species, a variety of heating regimes have been studied. To investigate electron heating regimes, mixtures of ³He and H were used in plasmas at 6.5T. Efficient (<80%) direct electron heating has been observed in this scenario and is attributed to mode conversion to the ion Bernstein wave (IBW). Measurements of the mode conversion power deposition using a nine channel grating polychromator (GPC) indicate that the IBW damping can be very strong, with central power densities >25MW/m³ and FWHM widths of ∼0.2a. The presence of >8% deuterium open-quotes impurityclose quotes in these plasmas is shown to significantly broaden the power deposition profiles. The GPC has also been used to study the heating mechanisms in the two standard C-Mod heating scenarios: D(H) and D(³He) minority heating at 5.3T and 7.9T respectively. Mode conversion can provide a significant fraction of the heating in D(³He) plasmas, with 60% efficiency and profiles which are peaked well off axis (r/a∼0.6) at the highest ³He concentrations (n_3He/n_e∼0.2). Data from D(H) experiments illustrate techniques to measure minority ion tails using electron temperature dynamics. In addition, evidence is presented for D(H) mode conversion heating at high hydrogen concentration. copyright 1997 American Institute of Physics