The subject of this work concerns the construction of a representation of a chemical reaction in macroscopic and microscopic forms by secondary-school pupils in the 15-16 year age range. All the surveys on this subject have revealed many unsolved difficulties among pupils. The balanced equation enables a chemical reaction to be represented in a microscopic form, but it can be read at a macroscopic level (that of the observed phenomena) or at a microscopic level (that of particles and the imagined phenomena). A study of the history of chemistry reveals that the necessity of representing a chemical reaction at these two levels presented chemists with an intellectual (epistemological) obstacle until the end of the nineteenth century. Our study revealed such difficulties among pupils and the recurrence and persistence of some of these difficulties enabled us to identify them as genuine epistemological obstacles which will require much work to overcome. Pupils must have educational experiences designed to allow them to move freely between these levels to appreciate the significance of the balanced equation. [Chem. Educ. Res. Pract. Eur.: 2000, 1, 61-75]