Nuclear singlet states are nonmagnetic states of nuclear spin-1/2 pairs that may exhibit lifetimes much slower than the relaxation of the component spins in isolation. This feature makes them attractive vehicles for conveying nuclear hyperpolarization in NMR spectroscopy and magnetic resonance imaging experiments and for reducing signal losses in other NMR experiments caused by undesirably fast nuclear spin relaxation. Here we show access to C-13(2) singlet states in a symmetrical oxalate molecule by substituting one or more O-16 nuclei by the stable nonmagnetic isotope O-18. The singlet relaxation time of the C-13(2) pair in [1-O-18,C-13(2)]-oxalate is 2-3 times longer than the spin-lattice relaxation time T-1.