A method of welding mitered surfaces of frame sections made of plastic material and having incorporated therein a seal of weldable material extending along the frame sections and composed of an attachment foot and at least a sealing lip, for use in windows or doors, includes the steps of heat-welding mitered surfaces and adjoining frame section zones by means of a heated mirror while covering during welding operation an inner contour of the seal by at least one form-part that is positioned in and neighboring the miter plane and has a contour which substantially complements the inner contour of the seal. Subsequently, after removing the heated mirror from the miter corner and withdrawing the form-part the area of the seal, the mitered surfaces of the frame section are squeezed together.