An experimental technique by which in-situ gas volumetric measurements are carried out on a neutron powder diffractometer, is presented and used for simultaneous studies of oxygen equilibrium partial pressure and the structural phase diagram of YBa2Cu3O6 + x. Experimental data was collected under near equilibrium conditions at 350 points in (x,T)-space with 0.15 < x < 0.92 and 25 °C < T < 725 °C. Precise values of oxygen stoichiometries (absolute values of x within 0.02) were obtained by use of the ideal gas law in connection with iodiometric titration and structural analyses. The temperature variations in lattice parameters were measured and used to establish the structural phase diagram. Pure tetragonal or orthorhombic phases were found. The (x,T) phase boundary for transition from tetragonal to orthorhombic symmetry gives strong support for a recent parameter free 2D Ising model calculation of the ordering of oxygen. Oxygen equilibrium partial pressure shows significant variations with temperature and concentration which indicate that x = 0.15 and x = 0.92 are minimum and maximum oxygen concentrations. Measurements of oxygen in-diffusion flow show relaxation type behaviour: View the MathML source. The prefactor j0 is activated with activation energies decreasing from 1.5 eV (x = 0.25) to 1.0 eV (x = 0.8). The relaxation time τ shows an anomalous behaviour that may be correlated with first order phase transitions predicted from theoretical model calculations.