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Objective: This study aimed to evaluate the effectiveness of two ion-releasing, resin-based composites (RBCs), ACTIVA BioACTIVE-Restorative (BIO1) and ACTIVA Presto (BIO2), in preventing demineralization around restorations., Methods: Class I and II cavities were prepared on bovine (n = 4) and human teeth (n = 8) and restored with BIO1, BIO2, a conventional composite (RBC, Filtek Supreme XTE), and a resin-modified glass ionomer cement (RMGIC, Ionolux). Following restorations (class I, n= 16/material; class II, n= 8/material), the specimens were exposed to Streptococcus mutans biofilm in a continuous-flow bioreactor over two weeks. Micro-computed tomography (micro-CT) assessed demineralization depths at restoration margins, and supernatant pH changes were measured after a 24-h acidic challenge. Statistical analyses included one and two-way ANOVA and Tukey's test (p < 0.05)., Results: On enamel surfaces, RMGIC showed no demineralization, followed by BIO1 (≈50 μm), BIO2 (≈125 μm), and RBC (≈150 μm). No difference between human or bovine enamel was observed. In dentin, RMGIC showed the least demineralization (≈190 μm), followed by BIO1 (≈230 μm), BIO2 (≈280 μm), and RBC (≈400 μm). pH buffering was highest in RMGIC (+ 0.24 pH), while BIO1, BIO2, and RBC showed similar buffering capacities (∼ +0.1 pH). Gaps were found at several interfaces for BIO1 and RMGIC., Conclusions: Ion-releasing RBCs varied in effectiveness for reducing demineralization of surrounding tissues, with limited pH buffering capacity. RMGIC exhibited better performance. Gaps between cavity walls and BIO1/RMGIC raised concerns about long-term adhesion., Clinical Significance: ACTIVA BioACTIVE Restorative (BIO1) and ACTIVA Presto (BIO2) resin-based composites promoted slight inhibition of demineralization in an in-vitro biofilm model, suggesting that further modifications in their chemical composition are necessary., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025. Published by Elsevier Ltd.)