It can be observed that, at 25 °C and for R ≥ 1.04, the x values are essentially independent of R and of the H2PP concentration lying in the range 0.59 ± 0.06. Surprisingly, even by increasing the reaction temperature to 60 °C, the x value keeps below 1, with less than half of the phosphate groups of ZrP exchanged with phenylphosphonate groups.