Partial oxidative gasification of n-hexadecane (n-C₁₆) and organosolv-lignin (lignin) was studied by use of a batch type reactor in supercritical water: 673 K, 0.52 cm⁻³ of water density (40 MPa of water pressure at 673 K), and 0.3 of O/C ratio for the n-C₁₆ experiments; 673 K, 0.35 cm⁻³ of water density (30 MPa of water pressure at 673 K), and 1.0 of O/C ratio for the lignin experiments. The experiments without O₂ were also conducted for lignin (lignin decomposition). For all the cases (n-C₁₆ partial oxidation, lignin decomposition, lignin partial oxidation), NaOH or zirconia (ZrO₂) was added in the system as catalysts. Through n-C₁₆ studies, the catalytic effect of NaOH and ZrO₂ on partial oxidation in supercritical water were examined. In the case of lignin partial oxidation, we studied the possibility of partial oxidation in supercritical water for gasification technique of wastes. The yield of H₂ from n-C₁₆ and lignin with zirconia was twice as same as that without catalyst at the same condition. The H₂ yield with NaOH was 4 times higher than that without catalyst. Thus, a base catalyst has a positive effect on partial oxidation of n-C₁₆ and lignin to produce H₂. The catalytic effect of NaOH and ZrO₂ was found to be enhancement of decomposition of intermediate (aldehyde and ketone) into CO, through n-C₁₆ studies. In the case of lignin studies, the enhancement of decomposition of the carbonyl compounds by catalytic effect of NaOH and ZrO₂ inhibit char formation and promotes CO and thus H₂ formation.