Abstract Electron spin polarisation achieved in the excited triplet state of quinoxaline diluted in p-dibromobenzene could be transferred to the bromine nuclear spins via forbidden Δ m S = 1, Δ m I = 1 transitions during an optical pumping cycle. A new experiment has been performed to destroy this nuclear spin polarisation in the ground state by nuclear quadrupole resonance on the 81 Br nuclei ( I = 3 2 ). This nuclear depolarisation was detectable as a significant change of the phosphorescence intensity of quinoxaline. The outstanding result is the improvement of sensitivity of NQR by three orders of magnitude. This way it was possible to investigate the diluted system of bromine nuclei near the substituted quinoxaline molecules. It turned out that these nuclei interact only weakly with the bulk bromine nuclei of the host material, presumably due to crystal distortions in the neighbourhood of the quinoxaline molecules.