A bioactive Si-Ca-P system (designated 58S) wet gel was prepared via a sol-gel method. The wet gel was reacted continuously in 0.25M K2HPO4 solution with a starting pH value of 9.0 at 37 °C for different time. The structural and compositional changes resulting from the conversion reaction were characterized using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and inductive coupled plasma spectroscopy. The results indicated that the induction period for the CDHA nucleation on the surface of the 58S bioactive wet gel was about 3 days. The conversion mechanism of the products was also discussed.