With increasing components variety in mixed-model assembly lines, kitting is an innovative line feeding mode to reduce congestion at the border of the line. This paper proposes a preliminary mathematical formulation of kitting activities for a hybrid robot-operator kitting system that delivers parts to a mixed-model assembly line. Indeed, elementary kitting operations are formulated, based on field assumptions, by distinguishing operations performed by the robot and the operator. Based on the formulation provided, the perspective of future research is to refine the model and to use the formulations in order to propose a quantitative model that would optimize the performance of the hybrid system.