It has been shown that polydimethylsiloxane (PDMS)-CaO-SiO2-TiO2 and poly(tetramethylene oxide) (PTMO)-CaO-TiO2 hybrids form apatite on their surfaces in a simulated body fluid (SBF) and show mechanical properties similar to those of human cancellous bones. In the present study, changes, caused by soaking in SBF, were measured in the mechanical properties of PDMS-CaO-SiO2-TiO2 hybrids with different CaO and TiO2 contents and PTMO-CaO-TiO2 hybrids with different CaO contents. Significant decreases in the strength and strain at failure of the hybrids were observed for the PDMS-CaO-SiO2-TiO2 hybrids with high CaO or TiO2 contents and PTMO-CaO-TiO2 hybrids with a high CaO content after soaking in SBF for 4 w. This indicates that incorporation of a large amount of CaO component into the hybrids should result in the deterioration of the hybrids in the body environment.