The structures of three adventitiously obtained peroxolanthanoid complexes have been determined, namely, [Yb2(Cp)4(μ-O)2/3(μ-O2)1/3(thf)2] (1) (Cp = cyclopentadienyl; thf = tetrahydrofuran), which has disorder between the bridging oxide and peroxide, [Nd2(o-PhPhForm)4(thf)4(μ-O2)] (2) (o-PhPhForm = N,N′-bis(2-phenylphenyl)formamidinate), and [Eu4(FForm)6(μ-OH)2(μ3-O2)2(μ-diglyme)2]·2diglyme (3) (FForm = N,N′-bis(2-fluorophenyl)formamidinate, diglyme = bis(2-methoxyethyl) ether). In the first two complexes, the peroxide bridges side-on between metals, whereas in the last complex, each peroxide bridges three metals through both oxygen atoms. The first complex was a single crystal amongst a bulk sample of [Yb(Cp)2(pzPh)(thf)] (pzPh = 2-(1′-pyrazolyl)phenyl), prepared by oxidation of dicyclopentadienylytterbium(ii) by bis(2-(1′-pyrazolyl)phenyl)mercury, the structure of which was also determined and showed distorted square planar stereochemistry for mercury.