A diffusion-controlled caulking treatment using aqueous colloidal silica to plug large pores in ceramic membranes is described. Unlike the conventional slip-casting process, this more controllable process can, in principle, be tailored by proper silica particle size selection to allow minimal deposition in the small desirable pores while enabling the elimination of large, less selective pores. The 2000 Angstrom Anodisc(R) membrane has been chosen as a model for the development of the procedure. Via the diffusion-controlled process, a majority of the 2000 A pores were closed using 100 to 200 Angstrom silica with negligible deposition on external membrane surfaces. The technique, therefore, is promising for selective closure of large undesirable pores without surface fouling or closure of desirable 100 Angstrom or smaller pores. The efficiency of the procedure has been characterized by a number of independent methods. The gas and water permeability of the caulked membranes decreased one and two orders of magnitude, respectively. Pore size analyses by a variation of the bubble point test and by rejection measurements of polyethylene glycol (MW 1500 to 12000) have found the average pore size of the dried silica-treated membranes to be approximately equal to the precursor silica sol particle size used in the treatment. While this suggests less than close packing in the large pores, the low permeability of the silica-treated membrane demonstrates that the treatment is highly effective. Indeed, given the selective caulking capability of this approach, it appears to be an attractive means of healing defects in microporous ceramic membranes.