A combination high‐sensitivity alternating current (ac) susceptometer and high‐frequencyB–Hlooper is described. Based on the well‐known inductance technique, the design uses modern low‐noise components and stable temperature control to achieve the high sensitivity. The mechanical assembly is designed for use inside a glass Dewar, enabling measurements to be made at any temperature between 4.2 and 300 K. Initial ac susceptibility can be measured over a frequency range of 10 Hz to 120 kHz with sensitivity of 1×10−8emu, comparable to a superconducting quantum interference device (SQUID) magnetometer.B–Hloops are measured at 2 kHz with a low duty cycle, damped oscillatory field pulse, and have sensitivity of 4×10−8emu. The instrument can thus measure ac susceptibility at higher frequencies than ac‐equipped SQUID magnetometers and commercially available inductive ac susceptometers, and can also measureB–Hloops at much higher frequencies and with greater applied fields than steady‐state loop tracers. ©1996 American Institute of Physics.