This paper describes the acquisition of conservation of matter by students aged 9‐15. Students were tested for their ability to recognize weight conservation as well as reversibility of process in the following changes in matter: Translocation, melting, dissolving and evaporation. It was found that children who recognized weight conservation in the translocation task did not necessarily recognize the same in the melting tasks, and those who recognized it in the melting tasks did not necessarily do so in the evaporation tasks. Students believe that a molten material weighs less than the same material in its solid form and that gas weighs less than the same substance in its liquid or solid form. In addition, students who recognized weight conservation were not always aware of the reversibility of the process. Until the age of 12 specific perceptual input from the task dramatically influences students’ responses to the conservation of weight tasks but not to the reversibility of process tasks.