Structure details of the incommensurate modulation in the Pb-doped Bi-2223 high Tcsuperconducting phase have been revealed by a multidimensional direct-method analysis of the electron diffraction patten of the (100) reciprocal plane. Occupational and positional modulations have been found for most atoms. Disordered oxygen atoms arranged perpendicular to the b axis bridging the Cu(2)-Ca-Cu(l)-Ca-Cu(2) layers were observed. This implies the existence of large amount of О-vacancies on the Cu(l)-0 plane. © 1992 IOP Publishing Ltd.