The amphiphilic chelating agent, n-octadecylacetoacetate (C(18)AA), plays a significant role in the two-dimensional sol-gel synthesis of titania and zirconia ultrathin films in improving the stability of the gel films floating on water surface and their deposition on a substrate by a Langmuir-Blodgett (LB) technique. The origin of the improvement effect of C(18)AA was investigated in detail by measurements of surface pressure-area isotherms and area-time curves as well as by IR and UV spectroscopy. C(18)AA molecules complex with the metal center of the gel film, and the resulting C(18)AA-stabilized gel film became stabilized owing the well-balanced. The effect of C(18)AA depends on the molar mixing ratio of metal butoxide/C(18)AA, x. In both the titania and zirconia systems, the quantitative LB deposition was possible when the x value was less than or equal to 3 and the amount of the gel transferred per one deposition became largest at x = 3. The x-dependence was satisfactorily explained from the relative amounts of the C(18)AA monolayer, C(18)AA-stabilized gel film and C(18)AA-free gel film on the water surface. The C(18)AA-stabilized gel films with x less than or equal to 3 deposited on substrates could be converted into the ultrathin oxide films of titania and zirconia by heating in air at 773 K for 0.5 h, and the thickness of the resulting oxide films was controllable in the range of sub-nanometer level not only by the number of deposition but also by the x value.