Monte-Carlo simulations are performed to investigate fluid-fluid phase equilibria and interfacial properties of nonadditive hard-sphere binary mixtures adsorbed in a slit pore. The behavior of critical density with wall separation and a nonadditive parameter is analyzed using mean field theory and simulations. The mixtures exhibit phase separation with the critical density rho(c) larger than its bulk counterpart. It is found that rho(c) decreases by increasing both the pore width H and the nonadditive parameter. The mean-field-like perturbation theory predicts rho(c) - rho(bulk)(c) similar to H-1; for large H and small rho(c), good agreement with simulation is found. The phase diagram, density profile, and interfacial tension are also obtained using different simulation methods. Interfacial tension, gamma, increases with the nonadditive parameter for a given total density and H. The effect of confining is to increase gamma for a given nonadditive parameter. A coupling between pore size and nonadditivity is discussed in connection with a possible spatially anisotropic separation. (C) 2003 Elsevier Science (USA). All rights reserved.