Dielectric barium strontium titanate films were deposited on MgO (001) substrate by pulsed‐laser deposition (PLD) and monitored in situ by means of reflection high‐energy electron diffraction and time‐resolved X‐ray diffraction (TRXRD). TRXRD showed two growth periods of the BSTO film and a transformation in the crystalline structure was detected as the thickness exceeds 80 nm. The occurrence of two different crystalline regions, namely BSTO1 and BSTO2 was proved by X‐ray diffraction (XRD). Ex situ transmission electron microscopy (TEM) techniques, including diffraction‐contrast as well as high‐resolution TEM, nanobeam electron diffraction, and scanning TEM in combination with energy‐dispersive X‐ray spectroscopy reveal structural and microchemical peculiarities of the BSTO film. By these TEM analyses, the presence of the two different regions BSTO1 and BSTO2 within the PLD‐grown BSTO layer was demonstrated. Regions of phase BSTO2 were found on top of nanoscaled MgO islands formed on the substrate surface during annealing at high temperature. While the majority phase BSTO1 has a single‐crystalline structure over wide ranges, BSTO2 regions seem to be poly‐ or even nanocrystalline, and the chemical composition of the two phases is also different. ... mehrThe transition in the growth periods is presumably related to the occurrence of BSTO2 regions during layer growth.