The influence of hydrogen gas on Fiber Bragg Grating (FBG)-based optical fiber sensors has been validated experimentally. More in particular, the focus was on FBGs written in the so-called Butterfly Micro Structured Fiber that targets simultaneous pressure and temperature monitoring with a minimum in cross-sensitivity to be used in, for example, downhole applications for the oil and gas market. The hydrogen-induced pressure and temperature errors from this type of sensor have been quantified as a function of the partial hydrogen pressure. The induced errors can be related to the diffusion of the hydrogen into the microstructure and to refractive index changes due to the presence of the hydrogen in the micro holes and penetration of it into the fiberglass. Furthermore, we have also shown that the hydrogen-induced errors scale with the partial hydrogen pressure.