Manganite is monoclinic, pseudorhombic. Its structure is a superstructure based upon the marcasite type. The crystals show polysynthetic twinning. By x-ray measurements on a thin untwinned cleavage splint improved Mn parameters were obtained. For the neutron investigations twinned crystals had to be used. Because of the polysynthetic twinning normally non identical reflections were superimposed, excepting ( hk 0), (0 kl ) and certain ( h 0 l ) reflections. From these reflections the H positions were derived. The neutron-diffraction measurement proved that the hydrogen is bonded to certain oxygen ions. High-temperature measurements showed that the superstructure exists up to the decomposition temperature of manganite. No coherent magnetic scattering was observed.