The syntheses and crystal structures of two cerium(IV) phosphonates are presented. Cerium(IV) bis(phenylphosphonate) Ce(O3PC6H5)(2) 1 can be formed from precipitation and mechanochemical reaction, whereas cerium(IV) bis(carboxymethylphosphonate) monohydrate Ce(O3PCH2COOH)(2) center dot H2O 2 is only accessible via ball milling. All reactions proceed very fast and are completed within a short time span. In situ measurements for the syntheses of 1 show that the product occurs within seconds or a few minutes, respectively. The structures were solved from powder X-ray diffraction data.