The category of mutuality that comes from Martin Buber’s dialogical philosophy is among those often discussed. What seems to be especially problematic is Buber’s maintaining that every I-Thou relationship is characterized by mutuality. Therefore the aim of the submitted paper is to clarify Buber’s understanding of mutuality as an integral part of I-Thou relationship and elucidate its function in this relation. The clarification is based on the distinction between two different meanings of the category of mutuality that can be found in Buber’s thought: There is a complete mutuality that depends on the attitudes of both relational partners as well as mutuality that depends just on the attitude of a man who enters an I-Thou relationship. The consideration of Buber’s basic principles is necessary in order to reject the first and more familiar understanding of mutuality as inadequate for the I-Thou relationship. On the other side, such understanding of mutuality as something independent from the attitude of man’s dialogical partner seems to be in accordance with these principles. At the same time, the distinction between the two meanings enables us to satisfy the question whether mutuality is really the basic characteristic of every I-Thou relationship.