Abstract: Method for studying constituents of individual molecular complexes by labelling the molecules belonging to the same complex with at least one set of molecular constructs, wherein each set member includes a Unique Identifying Sequence (UIS), which is a nucleic acid sequence unique for each set member, and at least one Common Tag Sequence (CTS), which is a nucleic acid sequence common to all set members, by: attaching the molecular construct to the complex by ligating or hybridizing the molecular construct to a nucleic acid molecule of the complex, or ligating or hybridizing the tag to a nucleic acid linked to an affinity binder that binds specifically to a constituent of the complex; labelling the molecules belonging to the same complex using the molecular construct tags as primers or templates in a nucleic acid polymerization reaction; and analyzing the composition of the complex by analyzing combinations of UISs and CTSs.