An investigation of temperature-induced instabilities in a wavelength measurement system based on macro-bend fiber filter used in the ratiometric scheme are presented. Twowavelength measurement systems based on macro-bend fiber, a standard low bend loss single-mode fiber filter based system and a high bend loss fiber filter based system are considered. In the case of a low bend loss fiber filter based system, the oscillatory variation in the ratio response with temperature and the difference in the temperature-induced ratio variation at different wavelengths makes the temperature corrected calibration of the system unfeasible, which is essential for precise wavelength measurements. The high bend loss fiber filter based system is more sensitive to temperature than the lowbend loss fiber filter based system, but critically for the high bend loss fiber filter based system the ratio has a linear variation with temperature, which can be corrected by a suitable temperature calibration process. Comprehensive studies on the temperature dependence of both the macro-bend fiber filter system and its effect on wavelength measurements are presented in this paper. �� 2009 IEEE.