A system for monitoring fuel additives on board a vehicle comprising; a fuel line configured to carry fuel from a fuel source to an engine; a fuel additive dispenser connected in parallel to the fuel line, wherein the fuel additive dispenser is a passive fuel additive dosing system; and at least one flow control device for controlling an amount of flow from the fuel line into the fuel additive dispenser; characterised in that the system further comprises: a fuel additive sensor configured to measure concentration of additives in fuel at a point along the fuel line; and a controller configured to receive input from the fuel additive sensor and to control the flow control device to adjust the amount of the flow from the fuel line into the fuel additive dispenser, wherein the passive fuel additive dosing system comprises a cartridge, a membrane-based contactor within the cartridge, a fuel inlet to the cartridge connected to receive diverted flow from the flow control device, a fuel outlet from the cartridge connected to return flow back to the fuel line, and an additive within the membrane-based contactor, the membrane-based contactor being arranged in the cartridge such that, with fuel in the cartridge, a fuel contact area with the membrane-based contactor is dependent on a fuel flow rate to passively dispense a proportional amount of the additive in to the fuel. a fuel line configured to carry fuel from a fuel source to an engine; a fuel additive dispenser connected in parallel to the fuel line, wherein the fuel additive dispenser is a passive fuel additive dosing system; and at least one flow control device for controlling an amount of flow from the fuel line into the fuel additive dispenser; characterised in that the system further comprises: a fuel additive sensor configured to measure concentration of additives in fuel at a point along the fuel line; and a controller configured to receive input from the fuel additive sensor and to control the flow control device to adjust the amount of the flow from the fuel line into the fuel additive dispenser, wherein the passive fuel additive dosing system comprises a cartridge, a membrane-based contactor within the cartridge, a fuel inlet to the cartridge connected to receive diverted flow from the flow control device, a fuel outlet from the cartridge connected to return flow back to the fuel line, and an additive within the membrane-based contactor, the membrane-based contactor being arranged in the cartridge such that, with fuel in the cartridge, a fuel contact area with the membrane-based contactor is dependent on a fuel flow rate to passively dispense a proportional amount of the additive in to the fuel. a fuel additive sensor configured to measure concentration of additives in fuel at a point along the fuel line; and a controller configured to receive input from the fuel additive sensor and to control the flow control device to adjust the amount of the flow from the fuel line into the fuel additive dispenser, wherein the passive fuel additive dosing system comprises a cartridge, a membrane-based contactor within the cartridge, a fuel inlet to the cartridge connected to receive diverted flow from the flow control device, a fuel outlet from the cartridge connected to return flow back to the fuel line, and an additive within the membrane-based contactor, the membrane-based contactor being arranged in the cartridge such that, with fuel in the cartridge, a fuel contact area with the membrane-based contactor is dependent on a fuel flow rate to passively dispense a proportional amount of the additive in to the fuel.