Hydrogen weight content of oleum, which is the mixture of H2SO4+ SO3, is correlated with oleum concentration in precisely described manner. Employing this correlation, concentration of oleum can be determined by measuring hydrogen content of oleum, by slowing down of fast neutrons. Investigations of prototype measuring head for measurement of oleum concentration, based on fast neutron slowing down principle, have been carried out. They indicate that achievable accuracy is 0,16%–0.53% of SO3content within measuring range 25% –65% of oleum concentration.