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Potentially bioactive sol-gel derived CaO-P2O5-SiO2 systems are studied and the influence of aggregation mechanism on the gel structure is discussed. A rheological measuring technique is used to monitor the aggregation process and the results are related to heterogeneity of the obtained gel structure. The results indicate that heterogeneity is produced in the sol-gel transition when calcium nitrate and phosphoric acid are used as precursors of CaO and P2O5. This destroys the correlation between bioactivity and the calcium and phosphorus content of the gel derived glasses.