Pb-Li alloy (Li 15.7-16.7at.%) is a candidate as tritium breeding blanket material for fusion reactors. Despite the extensive work reported in literature, the values of the Sieverts’ constant are spread over about two order of magnitude. The proposed Neutron Compton Scattering experiment aims at measuring the amount of hydrogen present into the liquid Li-Pb as different species (LiOH and solubilized hydrogen). The experimental conditions should approach as close as possible those of a liquid breeding blanket (T=500 °C, p=100 kPa). NCS may provide a double information: 1) estimate from the area of the H recoil peak the total amount of protons; 2) estimate the ratio of the bound protons (e.g. LiOH) to the solubilized ones, because of the large differences between the two species. DFT calculations will be used to provide accurate evaluations of in the two cases.