A method for production of high purity silica sols comprising the steps: (a) adding a phosphonic acid-based complexing agent to the silicic acid-containing solution forming complexes with metal cations present in said solution; (b) forming in said silicic acid-containing solution a precipitation containing phosphonic acid-based complexing agent and metal cations; (c) removing the precipitation from the solution; and, (d) polymerising the silicic acid in the solution to obtain a silica sol. The invention also relates to silica sols containing at least one phosphonic acid-based complexing agent.