We studied the Pr2O3/Si(001) interface by a non-destructive depth profiling using synchrotron radiation and photo-electron spectroscopy (SR-PES) at the undulator beam line U49/2-PGM2 and ab initio calculations. Our results provide evidence that a chemical reactive interface exists consisting of a mixed Si-Pr oxide such as (Pr2O3)(SiO)x(SiO2)y. There is no formation of neither an interfacial SiO2 nor interfacial silicide: all Si-Pr bonds are oxidized and all SiO4 units dissolve in the Pr oxide. Under ultrahigh vacuum conditions, silicide formation is observed only when the film is heated above 800°C in vacuum. Interfacial silicates like (Pr2O3)(SiO)x(SiO2)y are promising high-k dielectric materials, e.g., because they represent incremental modification of SiO2 films by Pr ions, so that the interface characteristics can be similar to Si-SiO2 interface properties. The Pr silicate system formed in a natural way at the interface between Si(001) and Pr2O3 offers an increased flexibility towards integration of Pr2O3 into future CMOS technologies.