Possibilities are explored for diagnosing the hot-electron distribution function of the thermal barriers of tandem mirrors, using perpendicular electron cyclotron emission. Emission from a relativistic bi-Maxwellian with a loss cone is calculated in the single-particle limit. Formulae are derived for finding T⊥ by measuring frequencies of the harmonio intensity maxima at optically thin frequencies, or the intensity at optically thick frequencies. A positive δf/δp⊥ in the distribution function is shown to lead to decreased absorption and instability for low T⊥. Finally, the perpendicular source function is calculated in the single-particle limit for this distribution function.