Kinetic parameters are reported for the oxidation of a range of substituted benzaldehydes, differing in their electronic and hydrophobic properties, as catalyzed by xanthine oxidase in different reversed micellar media. For uncharged substrates in water and in the three surfactant systems investigated (DTAB/hexanol/heptane/water, Triton X-100/hexanol/cyclohexane/water, and AOT/isooctane/water), the parameters k(cat) and K-MA and the ratio k(cat)/K-MA were correlated by linear free energy relationships (LFER’s). k(cat) was found to depend mostly on the substituent constant sigma, while 1/K-MA could be correlated with the partitioning constant pi. The proportionality constants to sigma and pi scaling the hydrophobic and electronic forces do not change significantly from one medium to the other, providing a strong argument for identical mechanisms and similar transition state structures in water and reversed micellar systems. The specificity ratio k(cat)/K-MA is lowered in reversed micelles compared to water. Medium effects on solvation were found to be on the order of 5-10 kj/mol, the same magnitude as substrate substituent effects. For charged substrates, the kinetic data fit a model allowing for partitioning of substrates between the two domains of water core and organic continuum. If the effective volume fraction available to charged substrates and products was restricted to be that offered by the water core, kinetic constants in water and in reversed micellar systems were found to match closely. It is concluded that reactivity in microheterogeneous systems such as reversed micelles is as accessible to analysis as reactivity in aqueous systems.