A sulfur breakthrough monitoring assembly for use in a fuel utilization system for detecting sulfur-containing compounds in desulfurized fuel, said monitoring assembly comprising: a heater adapted to heat desulfurized fuel that has been humidified to a predetermined temperature, said predetermined temperature being between 450°C and 600°C; a sulfur breakthrough detector adapted to receive heated fuel from said heater and including at least a reforming catalyst bed for reforming said heated fuel and a plurality of temperature sensors including a first temperature sensor adapted to sense a temperature of said heated fuel before said fuel is conveyed through said reforming catalyst bed and a second temperature sensor adapted to sense a temperature in said reforming catalyst bed; characterized in that the sulfur breakthrough monitoring assembly further comprises a controller adapted to: receive temperature outputs from the first and second temperature sensors (T3, T4), determine a plurality of differences between a temperature output of the first temperature sensor and a temperature output of the second temperature sensor over time, determine a rate at which the differences between the temperature output of the first temperature sensor and the temperature output of the second temperature sensor decrease, and determine that a concentration of the sulfur-containing compounds in the fuel exceeds a first predetermined concentration if the determined rate is equal to or greater than a first determined rate. a heater adapted to heat desulfurized fuel that has been humidified to a predetermined temperature, said predetermined temperature being between 450°C and 600°C; a sulfur breakthrough detector adapted to receive heated fuel from said heater and including at least a reforming catalyst bed for reforming said heated fuel and a plurality of temperature sensors including a first temperature sensor adapted to sense a temperature of said heated fuel before said fuel is conveyed through said reforming catalyst bed and a second temperature sensor a reforming catalyst bed for reforming said heated fuel and a plurality of temperature sensors including a first temperature sensor adapted to sense a temperature of said heated fuel before said fuel is conveyed through said reforming catalyst bed and a second temperature sensor adapted to sense a temperature in said reforming catalyst bed; characterized in that the sulfur breakthrough monitoring assembly further comprises a controller adapted to: receive temperature outputs from the first and second temperature sensors (T3, T4), determine a plurality of differences between a temperature output of the first temperature sensor and a temperature output of the second temperature sensor over time, determine a rate at which the differences between the temperature output of the first temperature sensor and the temperature output of the second temperature sensor decrease, and determine that a concentration of the sulfur-containing compounds in the fuel exceeds a first predetermined concentration if the determined rate is equal to or greater than a first determined rate. receive temperature outputs from the first and second temperature sensors (T3, T4), determine a plurality of differences between a temperature output of the first temperature sensor and a temperature output of the second temperature sensor over time, determine a rate at which the differences between the temperature output of the first temperature sensor and the temperature output of the second temperature sensor decrease, and determine that a concentration of the sulfur-containing compounds in the fuel exceeds a first predetermined concentration if the determined rate is equal to or greater than a first determined rate.