The reaction products of dibutylmagnesium with modified silica gel surfaces have been studied as relevant olefin polymerization catalyst precursors. Reactions were carried out on thermally and/or chemically pretreated silica gel. The pretreatments effect changes in silica surface silanol type and concentration. A combination of methods including infrared spectroscopy, NMR spectroscopy, and elemental analysis were used to characterize the chemically modified surfaces. Dibutylmagnesium reacts with non-hydrogen-bonded silanols to give a bond to the surface with one surface silicon. One butyl group is retained on magnesium. Dibutylmagnesium reacts with two hydrogen-bonded silanols to give bonds to the surface with two surface silicons. Both butyl groups are lost in the reaction with non-hydrogen-bonded silanols. Dibutylmagnesium also reacts with surface siloxanes to give a singly bonded magnesium, analogous to the reaction with non-hydrogen-bonded silanols. The adjacent silicon in the siloxane is believed to react with the second butyl group to form a silicon-butyl surface species. A majority of surface-reacted magnesium on the thermally pretreated silica gels studied is a result of siloxane bond breaking.