Aqueous oil-in-water emulsion comprising from 1 to 70% by weight, based on the total weight of the emulsion, of functional alkoxysilanes and/or condensed oligomers thereof and/or organoalkoxysiloxanes, at least one emulsifier and water, wherein the emulsion has a pH of from 5 to 9 and preferably a mean droplet size of the disperse phase of ≦0.5 μm, and the width of the droplet size distribution, expressed as the span (Formula I), is from 0.6 to 1.2. The preparation of the emulsions having the desired droplet size distribution is carried out in apparatuses having at least one pressure stage at a pressure of from 2 to 15 MPa. The emulsion can be used for hydrophobicizing porous mineral building materials.