A fuel characteristic determination system and method for determining one or more fuel characteristics of an aviation fuel for powering a gas turbine engine of an aircraft. The system includes a sensor component that is formed from a nitrile seal material and includes a surface that can be exposed to the fuel; a sensor for measuring a swell parameter of the seal material; and a fuel characteristics determination module for determining one or more fuel characteristics of the fuel based on the swell parameter. The method includes: exposing one or more seals of a fuel system of an aircraft to fuel within the fuel system; exposing a sensor component, made from the same material as the one or more seals, to the fuel; and measuring a swell parameter of the seal material.