A completely automated ac susceptometer is constructed to measure phase resolved linear and nonlinear ac susceptibility. Measurements over a wide range of experimental variables, such as ac and superimposed dc fields up to 300 Oe, frequency from 1 Hz to 100 kHz, and temperature from 77 to 300 K are made using only a lock-in amplifier (LIA), a PC, and some simple electronic circuits. A temperature-measurement and control unit is built utilizing the analog–digital input and digital–analog output of the LIA. Using a proportional and integral algorithm, a control accuracy better than 50 mK is achieved. The overall sensitivity of the setup is better than10−7emu and a reproducibility better than 0.1&percent; is achieved in the system. ©1997 American Institute of Physics.