Abstract: There is provided at least a kit for marking, monitoring and/or detecting alteration of a fuel comprising: (i) at least one marker; and (ii) at least one substance, wherein the substance is capable of interacting with at least one laundering agent. In particular, the kit may comprise a substance which is capable of interacting with at least one laundering agent to protect and/or prevent the marker from being absorbed, adsorbed, or partially or completely removed by the laundering agent. There is also provided a method of marking fuel, the method comprising marking a fuel with: (i) at least one marker; and (ii) at least one substance, wherein the substance is capable of interacting with at least one laundering agent.