Several techniques have been used to probe polymer end groups. The nitroxide radical trapping technique has been used (i) to show that initiator‐derived unsaturated end groups in polymethyl methacrylate can be minimized by using t‐hexyl peroxypivalate as the initiator (ii) to predict the end and penultimate groups in acrylonitrile/ethyl vinyl ether copolymer produced by t‐butoxyl initiation by analogy with the initiation mechanism (iii) to predict probable end groups in polyacrylonitrile and polystyrene produced by cyanoisopropyl initiation in the presence of adventitious oxygen. NMR techniques have been used to show that the end groups of functionalized oligomers, made from styrene and methacrylonitrile by the addition‐fragmentation chain transfer technique with allylic sulphides, conform to the expected structures.