The formation, precipitation, and deposition of so-called "magnesium silicate" in geothermal waters have been subjects of intense interest. Such scaling poses a severe threat to the smooth process of industrial systems. The aim of our approach is the systematic study of the influence of phosphonate-based chemical additives on silica polycondensation chemistry in the presence of magnesium ions. The focus of this work is the prevention of "magnesium silicate" formation using a variety of well-known phosphonate additives. These are 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), hydroxyethylidene-1,1diphosphonic acid (HEDP), aminotris(methylenephosphonic acid) (AMP), hexamethylenediaminetetrakis(methylenephosphonic acid) (HDTMP), and bishexamethylenetriaminepentakis(methylenephosphonic acid) (BHMTPAMP). Inhibition experiments were carried out in supersaturated solutions of silicate (200 ppm, expressed as SiO2) and magnesium (200 ppm, as Mg) at pH 10.0. The phosphonate additives used were found to act as stabilizing agents, most likely by complexing the Mg2+ cations and, thus, preventing "magnesium silicate" formation. On the basis of a plethora of experimental data, a number of useful functional insights have been generated, which add to building a more complete and comprehensive picture of the mechanism of "magnesium silicate" formation and stabilization.