The paper examines the kinetics of isobutane oxidation in the liquid phase and under supercritical conditions. Isobutane oxidation has gained importance because of the applications of the oxidation products such as tert-butyl hydroperoxide and tert-butyl alcohol in the manufacture of the important chemicals like propylene oxide and MTBE. An interesting aspect of this reaction is that, the reaction conditions can be conveniently manipulated in such a way that isobutane exists in supercritical state. The influence of supercritical conditions on the rate and selectivity of the reaction is investigated and this permits a comparison of liquid phase and supercritical phase oxidation. The reactions were also performed in a glass-lined continuous reactor. This mode of operation allows kinetic studies without an intervention of gas-liquid mass transfer and in addition eliminates the catalytic effect, if any, of the stainless-steel walls. The reaction is autocatalytic and the selectivity towards the hydroperoxide decreases with an increase in overall conversion. Under supercritical conditions, the rates and selectivity were significantly high compared to those obtained in liquid phase oxidation. In both, subcritical and supercritical oxidations, temperature has an adverse effect on selectivity towards tert-butyl hydroperoxide. A proposed kinetic model successfully explains the kinetic data and allows the rate constants and the activation energy to be determined. It is shown that the supercritical-phase oxidation shows all features present in the liquid-phase oxidation. Thus, this work provides a unified treatment for the supercritical- and the liquid-phase oxidation of isobutane.