The surface force apparatus has been used to measure the interactions between two mica surfaces immersed in aqueous solutions of dimethyldodecylphosphine oxide (DDPO). In addition, contact angles of dropets, from solutions with various concentrations of DDPO, on mica surfaces which had previously been immersed in the solution were determined. The aim was to explore to what extent a nonionic surfactant adsorbes onto a polar surface such as mica. These measurements indicate that DDPO adsorbs in a stepwise manner on the mica surface by forming two successive layers. The first layer is strongly adsorbed but the second layer can be removed by applying a large enough force. An adsorption mechanism for DDPO on mica is proposed. The possibility of a weak intetaction between the electronnegative oxygen in the head group and the proton adsorbed in the mica lattice is suggested.