In vitro mineralization of silicon containing calcium phosphate (Si-CaP) bioceramics was investigated in the constant-composition double-diffusion (CCDD) device under physiological conditions. The experiments were performed with porous pellets made of Si-CaP, which was synthesized from Ca(NO3)(2).4H(2)O, NH4H2PO4, ammonia and fumed silica according to a newly developed single-stage preparation route. Twice as condensed aqueous solutions of the Kokubo's revised simulated body fluid (rSBF) were used as mineralizing medium. All samples of Si-CaP bioceramics possessed excellent in vitro mineralizing properties and, therefore, this material is ready for further trials with cells. Simultaneously, the workability of the CCDD device in a less condensed (twofold instead of the previously used fourfold) aqueous solutions of rSBF was confirmed.