The authors have performed detailed high-pressure studies up to 110 kbar to investigate the nature of the structural phase of YBa₂Cu₃O₇ using energy-dispersive diffraction techniques. They observed a reversible orthorhombic to tetragonal phase transition at between 70 and 80 kbar. During compression it is found that the c axis has the highest compressibility; thus c/a decreases. A possible explanation for the relationship between the variation of structure with pressure and the transition temperature is given.