In situ neutron powder diffraction measurements show that the orthorhombic-to-tetragonal phase transition in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$, which occurs near 700\ifmmode^\circ\else\textdegree\fi{}C in a pure oxygen atmosphere, is an order-disorder transition in which the disordering of oxygen atoms into a normally vacant site destroys the one-dimensional Cu-O chains present in the room-temperature orthorhombic structure. For both structures, the oxygen stoichiometry decreases monotonically with increasing temperature. The transition temperature depends on the oxygen partial pressure and occurs when the stoichiometry is near Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.5}$. The tetragonal structure has a partially occupied, nearly octahedral Cu-O arrangement, in contrast to the orthorhombic structure which has one-dimensional Cu-O chains. The observed depression of the superconducting transition temperature in tetragonal $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$, which has been quenched from high temperature, could result either from the disordering of oxygen atoms which destroys the one-dimensional chains or from the absence of ${\mathrm{Cu}}^{3+}$ ions.

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