A new chemical method, differential dissolution, has been used to characterize thin (0.05-1 mu m) YBCO films. By this technique the qualitative and quantitative phase composition of the films was determined. New information on the film inhomogeneity was obtained. Chemical inhomogeneity was defined as second-phase precipitates at a content below 10(-6) g and as local deviations from 123 stoichiometry of the main phase throughout the thickness of the film. Not only the total quantity but also the interphase distribution of Al in poly- and single-crystal films with the sapphire substrate temperature were established. Occurrence of amorphous or disorder forms of the 123 phase together with the crystalline one, as an effect of the substrate temperature was detected and quantities of the forms were correctly determined. The obtained information is considered in combination with structural parameters and transport properties of the films. The technique as an important tool for analyzing thin films and its application to growing the single-crystal YBCO films by laser deposition are discussed.