Investigations into the structure of cyclotron resonance absorption signals of free electrons in the gas phase have been extended. Experiments in which electrons were produced by chemi-ionization reactions of oxygen atoms with unsaturated hydrocarbons or by microwave discharges in inert gases show that the structure of the signals is largely independent of the nature of the gas and the mechanism of electron production, but depends on the detection power and the position of the sample in the electron paramagnetic resonance (e.p.r.) detection cavity. Existing theories have been tested and found to be unsatisfactory. The results suggest that the origin of the structure lies in the interaction of the electrons with the microwave field in the detection cavity but a detailed theory cannot yet be formulated.