Experimental evidence is presented to verify the quantitative predictions of interfacial defect theory as applied to strain relief in epitaxial PbTiO3thin films through the formation of 90° domains. Epitaxial PbTiO3thin films grown by metal‐organic chemical vapor deposition on MgO(001), SrTiO3(001), LaAlO3(001), and SrRuO3(001)/SrTiO3(001) substrates are examined using four‐circle x‐ray diffraction and transmission electron microscopy. The data represents a detailed examination of the ...c/a/c/a... 90° domain patterns that develop during the paraelectric to ferroelectric (PE→FE) phase transition as the film is cooled from the growth temperature. Three independent measurements of the relative coherency strain (er) are reported. The data quantitatively and self‐consistently verify the crystallographic rotations predicted by the concept of the relative coherency strain and demonstrate the validity of domain stability maps in understanding the phase transformation history in epitaxial ferroelectric thin films. ©1996 American Institute of Physics.