Abstract: A multi-additive injection system for injecting required amounts of different additives to flowing fuel in a fuel line (6) comprises a number of additive injection sub-systems (1, 2, 3 and 4). Each sub-system includes a source of additive (7), a pump (13) to deliver additive from the source (7) to the fuel line (6), and an additive meter (21) to measure an amount of the additive being delivered to the fuel line (6) by the pump (13). The multi-additive injection system further includes a meter to measure the amount of fuel flowing through the fuel line (6) and a control device such as a computer to control the pump (13) of each sub-system to adjust the amount of additive being delivered by each pump, should the amount metered by the meters (21) deviate.