The nucleation of the c-axis aligned and non-c-axis Yba(sub 2)Cu(sub 3)O(sub 7(minus)(delta)) (YBCO) from precursor films on (001)-cut SrTi(sub 3) was investigated for the so-called BaF(sub 2) process. Specimens with different thickness were quenched from 735 deg C, then studied by transmission electron microscopy and x-ray diffraction techniques. Preceding the formation of YBCO nuclei, three intermediate phases of (Y,Ba) oxy-fluoride and a transition phase without F were found in the precursor films. These were structurally and chemically related to the nuclei of YBCO which was found to be deficient in Cu relative to its stoichiometric composition.