A system for the direct and continuous determination of the irreversibility line in high Tc superconductors is described. The system, based on a standard ac susceptometer, operates by applying a controlled dc field to the sample that maximizes the imaginary (out-of-phase) component of the ac susceptibility. This control is achieved using a proportional-integral-derivative loop with the reference set to zero and with the magnetic field derivative of the imaginary component of the ac susceptibility taken as the feedback sample (error signal). To obtain a dc voltage proportional to this derivative a double lock-in detection scheme is used. This apparatus is able to carry out a continuous measurement of this line by sweeping the temperature in the range of interest while the sample is maintained, by the controller, at the irreversibility magnetic field at each temperature. Hence, the whole process is accomplished in the same time as a typical resistivity-versus-temperature measurement, which is faster than other methods. © 1998 American Institute of Physics.