In the present study, the analysis of students' conceptions about chemical reaction, based on the theoretical framework of Vosniadou about mental models, is described. Students' conceptions were found out by a former research dealing with the knowledge and cognitive skills that the students obtain from the Chemistry courses. The sample of research was constituted of 499 9th-grade and 624 11th-grade students from schools of four prefectures: Attica, Boeotia, Heraklion (Crete), Thessalonica. The most important results of this analysis are that from the students' conceptions about chemical reaction, except of the scientific model of chemical interaction, four more mental models are derived: (a) displacement, (b) change of form, (c) mixing of substances, and (d) transformation of a substance. These four models are constrained by the ontological presuppositions that the substances are stable physical objects, and that the object's identity is determined by a set of observable properties.