We show that, using the eigenstates of a high-finesse Fabry–Perot cavity, we can measure the small Cotton–Mouton birefringence of gases with a simple tabletop apparatus. It requires only modest magnetic fields(B<1 T)and uses an ultrastable Zerodur high-finesse optical cavity. The method is validated by the measurement of the Cotton–Mouton constant of air and a sensitivity of2.5×10−9 radis obtained for the single pass birefringence. ©1998 American Institute of Physics.