In reactance-tube frequency modulation circuits, the fractional frequency deviation is expected to vary with the phase shift constants of the modulator and the Lf/Cf ratio of the tank-circuit of the oscillator. The way in which the frequency deviation is affected by these factors is discussed theoretically. Experimental checks are made, Two types of circuits are studied, the inductive and the capacitive reactance-tube circuits. It is found that within the straight portion, of the operating characteristic of the tube, the agreement is fairly satisfactory.