The hydrolytic condensation of organotrialkoxysilanes containing (beta-hydroxy) tertiary amine groups generates a significant fraction of intramolecular cycles and leads to the formation of closed structures. The aim of this study was to investigate if their co-condensation with a tetraalkoxy silane would take place by predominant intramolecular cyclization steps, eventually leading to functionalized silicas that can be dissolved in appropriate solvents. Co-condensation of tetraethoxysilane (TEOS) with the organotrialkoxysilane derived from the reaction of N-(beta-aminoethyl)-gamma-aminopropyl trimethoxysilane with phenyl glycidyl ether, was performed in the presence of HCOOH using a thermal cycle attaining 150 degrees C. For TEOS molar fractions lower than 0.7, soluble functionalized-silicas containing up to about 25 wt% of inorganic silica chemically bonded to bulky organic groups, could be synthesized. The glass transition temperature of the organic part was close to 25 degrees C it was inferred that the inorganic part was vitrified at the maximum temperature of the selected thermal cycle (150 degrees C). Most of the Si atoms supplied by the trialkoxysilane were present in fully condensed structures (T-3), while those supplied by TEOS were present mostly as Q(3) and Q(4) structures. These soluble functionalized-silicas may be used to introduce silica as a reinforcement of a polymer network. An example is shown, using an epoxy-diamine formulation. The equivalent of 10 wt% silica could be incorporated to the final network at the expense of a reduction of 26 degrees C in its glass transition temperature. (c) 2005 Elsevier Ltd. All rights reserved.