A new approach to the necessity analysis method using simultaneously combined sensitivity and reaction flow analysis through each reaction is proposed, validated and applied in this paper. Principles of the necessity analysis method and developments in the new approach are given in detail. Validity of the new approach is investigated by the reduction of considerably small reaction mechanism, i.e., methane mechanism, for the CPOX (catalytic partial oxidation) for the homogenous gas phase in the DETCHEMBATCH code. After the validation, it is applied for the CPOX of iso-octane, which was developed by the Lawrence Livermore National Laboratory (LLNL), for the homogenous gas phase for the same conditions. It is seen that necessity analysis method allows deleting up to 53-56% of the species, from the detailed mechanism without much loss information, for the considered conditions (C/O = 1.6 and C/O= 1.2). The new approach allows deleting 63% of the species for the same cases. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.