This paper addresses memorization processes and functioning of memory abilities. The aim is to explore the dynamics of memory abilities with the increased difficulty of the task. We have attempted to uncover qualitative and quantitative characteristics of the system of mental operations used to memorize new material. The method suggested helped not only to identify the stage of immediate memorization and inclusion of mental operations into activity but also to uncover the subsequent development of the mental operations system in the process of memorization. We have studied the significance of qualitative and quantitative characteristics of the mental operations system of memory abilities. It has been concluded that memory activity includes different types of mechanisms, i.e. immediate memorization and memorization with the use of the system of mental operations. The link between those mechanisms has also been identified.