A complex photothermal refractive index change in host-guest liquid crystals was determined with a novel interferometric method. The complex refractive index change was proportional to the pump beam intensity. The absolute value of the real part was about ten times as large as that of the imaginary one. The absolute value of the extraordinary refractive index change was about nine times as large as that of the ordinary one. The real part of the extraordinary refractive index change was negative, while that of the ordinary one was positive, and the imaginary part of both the extraordinary and ordinary refractive index change was negative. These results can be explained by the photothermal effects in the host-guest liquid crystals and dyes. ©1997 American Institute of Physics.