The present invention relates to methods for linking tetrazines with dienophiles to establish at least two linkages by sequentially performing at least two cycloaddition reactions. The methods in particular allow establishing multi-labeling strategies. In particular, the invention relates to methods for forming linkages by cycloaddition reactions, wherein the method comprises reacting a first alkyl-substituted tetrazine with a first dienophile comprising a irans-cyclooctenyl group followed by reacting a second tetrazine with a second dienophile comprising a cyclooctynyl group, wherein the reaction of the first tetrazine with the first dienophile proceeds in the presence of the second dienophile.
