The crystal structure and nonstoichiometry of the mixed Cu-doped lanthanum manganates LaMn1-zCuzO3+/-y (0 less than or equal to z less than or equal to 0.4) have been investigated by powder X-ray diffraction (PXD) and powder neutron diffraction (PND). The samples quenched in air from 1173 K to room temperature were crystallized in the orthorhombic structure (space group Pbnm). The oxygen positions, interatomic distances and occupancies have been refined based on the PXD and PND data. According to the refinement, mixed oxides LaMn1-zCuzO3+/-y with 0 less than or equal to z less than or equal to 0.2 exhibit full occupancies of the oxygen sites (O1 and O2) and deficiency of the La-sites. Oxygen nonstoichiometry in LaMn1-zCuzO3+/-y samples with values of z ranging from 0 to 0.3 were studied as a function of temperature (1150-1450 K) and oxygen partial pressure (10(-3.5)-1 atm) by means of thermogravimetry (TG). The absolute values of the oxygen nonstoichiometry (y) were determined by a full reduction in hydrogen and iodometric titration. A defect model describing experimental data is discussed.