It is demonstrated that a careful X-ray diffraction analysis represents an effective way to determine imperfections and their density in complex oxide epitaxial thin films. A method for simulating X-ray intensities of the (00l) reflexes is developed and demonstrated for the model system YBa2Cu3O7-delta- In a first step, the delta dependence of the intensities of the different (00l) reflexes is simulated and compared to literature data of perfect YBa2Cu3O7-delta thin film and bulk samples with different oxygen content. In a second step, it is demonstrated that the 6 dependence of the intensities of the different (00l) reflexes depends strongly on the type of defects in case of imperfect YBa2CU307-6. Different types of defects (e.g., cation disorder, cation substitution, Cu deficiency) are discussed. Finally, the method is applied to low-pressure sputtered YBa2CU307-6 thin film. It is shown that according to this analysis and in contrast to previous assumptions, Cu(I) deficiency seems to be responsible for the elongation of the c-axis and the reduction of the superconducting transition temperature. (c) 2006 Elsevier B.V. All rights reserved.