To predict the structures, properties, and chemistry of materials involving silicon and silicon oxides; interfaces between these materials; and hydrolysis of such systems, we have developed the ReaxFF(SiO), reactive force field. The parameters for this force field were obtained from fitting to the results of quantum chemical (QC) calculations on the structures and energy barriers for a number of silicon oxide clusters and on the equations of state for condensed phases of Si and SiO2 from QC. We expect that ReaxFF(SiO) will allow accurate dynamical simulations of bond breaking processes in large silicon and silicon oxide systems. ReaxFF(SiO) is based closely on the potential functions of the ReaxFF(CH) reactive force field for hydrocarbons, so that it should also be useful for describing reactions of organics with Si and SiO2 systems.