A sensitive submillimeter heterodyne receiver has been used to measure electron cyclotron emission at the 2nd through the 5th harmonics from thermal plasmas in Alcator A. A sensitive diagnostic of the electron energy distribution function is obtained from intensity ratios of the optically thin harmonics, in particular the small runaway fraction (∼10−6) that exists even in well thermalized Alcator A plasmas. Also the 4th and 5th harmonics are a sensitive measure of impending plasma disruption due to vessel air leaks.