A study of the sub-system Ca2SiO4-7CaOP2O52SiO2 within the system Ca3(PO4)2-Ca2SiO4 was conducted as a preliminary step toward obtaining new biomaterials with controlled microstructures. Phase composition of the resulting ceramics was studied using X-ray diffraction and scanning electron microscopy with attached wavelength dispersive spectroscopy. The results showed that the sub-system presents an invariant eutectoid point at 518 degrees C +/- 6 degrees C with a composition of 31 wt% Ca3(PO4)2 and 69 wt% Ca2SiO4, and typical eutectoid microstructure of lamellae morphology. These results are in disagreement with the previous reported data, which locate the invariant eutectoid point at 500 +/- 20 degrees C with a composition of 36.75 wt% Ca3(PO4)2 and 63.25 wt% Ca2SiO4.