The initial dissoln. behavior of glasses based on bioactive glass S53P4 was studied with a dynamic measurement setup in a Tris-buffered soln. The glass compn. was modified systematically on a molar basis by replacing sodium oxide with potassium oxide (0-100% K) and calcium oxide with magnesium (0-18% Mg) or strontium oxide (0-100% Sr). The concns. of the ions dissolving from the glasses were measured continuously online in the fluid flow for 15 to 25 min using an inductively coupled plasma emission optical spectrometer. This method enabled attainment of detailed information on the initial dissoln. mechanisms without the, for bioactive glasses typical, interference of apatite layer formation. The results showed that initial dissolns. of sodium and potassium were markedly higher from the mixed alkali oxide glasses than from the compns. contg. only one alkali oxide. Introducing MgO in S53P4 caused a minor decrease in the dissoln. rates of all ions. The glass contg. 3 mol% of MgO showed the best chem. durability. In contrast, replacing CaO gradually with SrO increased the dissoln. rates of all ions. The glasses with the highest replacement of CaO with SrO showed rapid release of both Sr and Na ions. The results corroborate the overall knowledge of glass durability and can be utilized to design bioactive glasses with controlled ion release rate for tissue engineering applications.