The problem of the role of activities in chemical rate equations is analyzed. The Mass Action Law - based formulation with concentrations and the Thermokinetic chemical potential -based formulation are consistent only in ideal systems. A new interpretation is presented for non-ideality using the concepts of ＇＇fractions＇＇: in a frame of three-level structure of physical chemistry. The fractions are the physico-chemical components of the system between which only ideal relationships exist. It is shown that activities really are concentrations of particular fractions of the chemical components. In non-ideal mixtures the Mass Action Law is to be formulated with the fractions as participants in the elementary process instead of the chemical components. In this case the contradiction activity - concentration is dissolved.