The present invention relates to modified phenolic resins which contain silicic acid ester units. The modified phenolic resins may, for example, be used as components of a foundry binder system. The invention also relates to a method for producing such modified phenolic resins and to two-component binder systems which contain these modified phenolic resins. The invention furthermore relates to methods for producing foundry molds and foundry cores which contain the modified phenolic resins, and to the foundry molds and foundry cores themselves.