Alcator C-Mod uses high power radio frequency (rf) power for auxiliary plasma heating at the ion cyclotron frequency. The operation of the rf antennas is limited at neutral pressures which fall far below the minimum pressure for glow discharge as given by the rf Paschen curve. Recent experiments have found that this operational pressure limit is due to multipactor discharges in the vacuum transmission lines. At elevated neutral pressures, the multipactor discharge induces a glow discharge that causes a nearly complete reflection of rf power. On C-Mod, in presence of a magnetic field, this discharge forms at pressures of 0.5 and 1 mTorr, which are the same pressures as the observed operational limit. Experimental results are supported by the Coaxial Multipactor Experiment. (c) 2006 American Vacuum Society.