We report on mixed surface micellization behavior of two diblock copolymers: PS-b-P2VP (M-w = 56 000, w(PS) = 50%) and PS-b-PMMA (M-w = 50 000, w(PS) = 50%). Both diblock copolymers were synthesized by sequential anionic polymerization and have narrow molecular weight distributions. PS-b-P2VP and PS-b-PMMA were spread at the air-water interface in dilute chloroform solution (0.5 mg/mL) by two different methods: (1) spreading of a premixed solution and (2) separate spreading of each block copolymer solutions sequentially. The pi-A isotherms were measured at different subphase pH, and the morphology of the surface micelles was investigated by atomic force microscopy after transferring to a Langmuir-Blodgett film onto a silicon wafer. The ionization of the P2VP block induced different morphology at low pH from which the aggregated structure of block copolymers in micelles could be delineated. When PS-b-P2VP and PS-b-PMMA are spread as a premixed solution, two block copolymers form surface micelles sharing a PS core and mixed corona of P2VP and PMMA chains while independent PS-b-P2VP micelles and PS-b-PMMA micelles coexist when the block copolymers were spread separately. These results indicate that the surface micellization takes place rapidly after spreading, and there is no observable exchange of the block copolymers between the micelles in a reasonable time span.