The electrical treeing resistance of epoxy-SiO2 nanocomposites prepared by synthesizing functionalized SiO2 nanoparticles directly in epoxy was investigated and compared to pure epoxy (diglycidyl ether of bisphenol-A). Partial discharge measurements indicate a transition of initially non-conducting trees to conducting trees in both the pure epoxy and the nanocomposites. A correlation between the synthesis precursors and the partial discharge behaviour over time indicates that the presence of ions and remnant precursors hasten the transition to conducting trees and lower the voltage of tree inception. The resistance to tree growth was found to be dependent on the synthesis conditions of the nanocomposites. Slower tree growth was observed in nanocomposites prepared under neutral conditions, with a homogenous dispersion of SiO2 nanoparticles (30-50 nm, 5 wt%). The presence of ions from alkaline conditions and/or poor dispersion of SiO2 resulted in faster and more linear tree growth and lower initiation voltages than for pure epoxy.