During mechanical activation, the energy of treated material is increased to a higher level. This can lead to the chemical transformation of the activated material. This is the point where we can talk about mechanochemical transformations that have occurred as a result of mechanical activation. The subject of this paper is to monitor mass changes of material after different degrees of activation. One of the substances which are often used in the processes of mechanochemical synthesis is sodium carbonate. The mass changes occurring during the treatment were detected and measured by various methods, depending on the processing environment. The mass increase was attributed to the chemisorption of moisture and carbon dioxide present in the air, as a consequence of the sodium carbonate activation. The methods we used were calcimetric chemical analysis. According to obtained results, it was found that activated sodium carbonate is mass-transformed into sodium bicarbonate, whereby these changes are functionally dependent on activation time and the processing atmosphere.