The magnetic susceptibility of coadsorbed O-2 and CO2 confined in a slit-shaped graphitic micropore was measured over the temperature range 2-300 K. Coadsorbed CO2 markedly repressed the susceptibility of confined O-2 because CO2 restricts the O-2 molecular arrangement to form spin clusters. Curie-Weiss plots show that the coadsorbed CO2 reduced the effective spin concentration while the negative Weiss constant with CO2 was larger than that of pure O-2 These results also indicate that the coadsorbed CO2 promotes the formation of smaller clusters of O-2 molecules than for pure O-2.