The present invention provides a bioconjugation method and compounds for use therein. The bioconjugation method comprises the step of conjugating a biological molecule containing a first unsaturated functional group with a payload comprising a second unsaturated functional group, wherein the first and second unsaturated functional groups are complementary to each other such that conjugation is a reaction of said functional groups via a Diels-Alder reaction which forms a cyclohexene ring.