Growth and thermal strains represent main contribution to the total strain in the majority of thin films. The main aim of this work is to demonstrate the possibility to quantify growth and thermal strains/stresses and to detect the presence of other strain-contributing phenomena in epitaxial thin films using elevated-temperature X-ray diffraction measurements. In the first part, a mathematical formalism is derived allowing to refine different strain/stress phenomena from the total strain. In the experimental part, X-ray diffraction is used to evaluate temperature dependencies of strains/stresses in GaN thin films deposited on sapphire. The approach allows to determine temperature dependencies of stresses, extrapolate growth stresses, temperatures of stress-free state, annealing-induced relaxation of stresses and other important physical parameters.