Abstract: A method is described for compensating measurement errors of fiber Bragg grating (FBG) sensors installed in fiber cables in down-hole oil and gas environments due to hydrogen darkening. The method involves loading a fiber of known length and multiple FBG sensors in a hydrogen chamber with control of temperature, hydrogen concentration, pressure, and time and adjusting each of the variables through a test matrix while continuously measuring and recording the fiber transmission loss and FBG spectrum changes, then creating a correlation map of fiber transmission loss vs. Bragg wavelength shift, and using that map for compensating the wavelength shift of downhole installed fiber cables as a function of their hydrogen induced transmission loss through time.