The magnetic structure of two well characterised YBa 2Cu 3− x Li x O y samples, with similar Li concentrations, x=0.08–0.09, and slightly different oxygen concentrations, y=6.06 and y=6.18, has been studied by neutron powder diffraction measurements. The lithium substitution is found to induce a strong decrease of the Néel temperature and of the mean ordered magnetic moment on the Cu2 sites. T N is ≈245 K for y=6.06 instead of 415–400 K in unsubstituted YBa 2Cu 3O y with y=6.0–6.2. The mean ordered moment at low temperature, μ 0/ μ B, is 0.33±0.03 for y=6.06, and 0.17±0.08 for y=6.18, instead of 0.64 in unsubstitued YBCO. These measured values are compared with other data from the literature and the various possible mechanisms of the strong lithium effect are discussed.