Muonium atom addition to cyclopent-4-ene-1,3-dione I leads to the adduct radical II; in contrast with all other muonium adducts of carbonyl (CO) compounds so far studied, in which the isotropic muon coupling is of positive sign, the temperature dependence of the muon coupling in II reveals that its sign is negative; this may be explained as a consequence of the electronic ‘push–pull’ interaction within the MuO–C–CC–CO system, which reduces the out-of-plane vibrational amplitude of the muon by increasing the MuO–C/MuOC partial π-bond character.