The oxidation of isobutane has been investigated for the first time in a micro reactor for a wide range of process conditions comparing the product ratios obtained with di-tert-butyl peroxide (DTBP) and tert-butyl hydroperoxide (TBHP) as initiator. DTBP led to nearly the double conversion compared to the same concentration of TBHP, but the selectivity of TBHP using TBHP as initiator was higher. Using aqueous TBHP, the conversions reached up to 6 % in case of supercritical conditions but stayed below 2 % in case of two-phase flow experiments. With TBHP in decane as initiator a high conversion was reached but nearly no TBHP was formed. It seems that the silicon layer of the micro reactor was not stable under such experimental conditions. In the present work, for the first time a conversion level near to that of the industrial batch process has been reached in a micro reactor for the isobutane oxidation.