A monitoring assembly for use in a fuel cell system for detecting sulfur-containing compounds in fuel. The monitoring assembly comprises an indicator assembly for passing the fuel therethrough, the indicator assembly including an indicator material and a housing for housing said indicator material, wherein the housing is adapted to be placed on-line in one of a main path receiving substantially all the fuel and a bypass path receiving only a portion of the fuel in the fuel cell system and the indicator material is such that when the housing is placed on-line in the fuel cell system at least one physical property of the indicator material changes when the indicator material is exposed to sulfur-containing compounds in the fuel of the fuel cell system, and the indicator assembly being additionally adapted to allow detection of the change in the physical property of the indicator material.