Abstract: The stacking order of a 14.30-Å Mg-vermiculite from Santa-Olalla, Spain, has been determined from Weissenberg photographs. The results prove that the Mg-vermiculite structure is not a 2-layer polytype structure, but a semi-ordered one. Because the structure is semi-ordered, its resolution needed a dual approach: (1) a direct approach using an electron density projection along the y axis in conjunction with a one-dimensional electron-density projection onto the z axis; and (2) an indirect approach in which the observed intensities along the (0,k) and (1,k) reciprocal rods were compared to the calculated intensities given by model defect structures. The semi-ordered structure of the Mg-vermiculite results from ±b/3 shifts in the stacking of the silicate layers. The shifts are randomly either along +b or -b.