Abstract Objectives. An extensive number of investigations have focused in recent years on the process of fluoride release by glass–ionomer cements. In order to compare the fluoride release of two recent resin-modified glass–ionomer cements (RMGICs) and one compomer, we investigated the initial and subsequent fluoride release in distilled water and in a mineral medium with composition similar to saliva (SAGF), as well as the renewal effect of the media on the release. Method. A preliminary thermodynamic study defined conditions under which the measurements by fluoride-selective electrode potentiometry were not affected by the presence of Ca 2+ ions in SAGF. Disk-shaped samples (5 mm×3 mm) for each material were placed in 5 ml of distilled water ( n=12) or 5 ml of SAGF ( n=12) and fluoride released was measured after 1, 2 and 7 days. For the investigation of medium renewal effect, RMGIC samples were dipped into distilled water for 5 days. Subsequently 12 samples of each material were placed for 7 days into water or SAGF, with or without daily renewal of the medium before fluoride-release measurements. The elements present on the surface of the RMGICs were analyzed by scanning electron microscopy. Results. RMGICs released significantly more fluoride in water than in artificial saliva. The presence of CaF 2 at the interface accounts for this phenomenon. The thickness of the CaF 2 layer depends on the speed of its initial formation and renewal of the testing medium. A pattern of CaF 2 formation was proposed.