Susceptibility measurements, &khgr;′, can be precisely and simply performed in terms of the relative frequency shift (&Dgr;&ngr;/&ngr;) of a rf self‐oscillator, occurring when the permeability of the space is changed by the introduction of the sample. The essential feature is the transfer of the sensitive volume from the inductor to a narrow gap of a ferrite toroid. When &Dgr;&ngr; is a periodic function, standard frequency modulation detection is employed. The calibration of the apparatus against known diamagnetic and paramagnetic substances indicates a sensitivity &Dgr;&ngr;/&ngr;0= 1.6 × 10−8(which corresponds to a minimum detectable value of the susceptibility per unit volume of 3 × 10−9emu, accessible with a sample volume as small as 0.1 cm3). It is also shown that the minimum &Dgr;&ngr;/&ngr; can be further decreased with respect to the experimental condition of the present measurements.