An approximate theory is developed of the breakdown characteristics of a coaxial diode in an axial magnetic field, taking into account the effects of elastic collisions. It is assumed that the electron moves in a constant electric field between collisions and thus the theory is valid only in the appropriate range of magnetic field and voltage. Estimates of transit time and of space-charge effects are also made. Measurements in the pressure range 10−3 to 10−9 mm. Hg are in general agreement with the theory.