When labelled with two C nuclei, the small, highly symmetrical squarate molecule is an attractive candidate for exhibiting long-lived C 2 singlet order in aqueous solution (see figure ). The squarate dianion is stabilised by electron delocalisation and exhibits four-fold rotational symmetry . Since the dominant oxygen isotope O has no nuclear spin, the C pair of 1,2-13 C 2 -squarate is magnetically isolated, eliminating all intramolecular dipole-dipole contributions to singlet relaxation. Furthermore, it is technically feasible to introduce non-magnetic O nuclei into the squarate dianion, as has been achieved previously for oxalate . The increased mass of the O nuclei modifies the vibrational wavefunctions, inducing small secondary isotope shifts of the 13 C resonances . These shifts may break the exact magnetic equivalence of the C nuclei, providing access to long-lived C 2 singlet order. Large values of the singlet relaxation time constant T S are therefore anticipated for the C pair in O-enriched 1,2-13 C 2 -squarate, especially at low magnetic field, when the chemical shift anisotropy mechanism of singlet relaxation is removed.