This study was conducted to investigate the conditions used to prepare Pt/TiO2 catalysts via a liquid-phase reduction method and to analyze the room-temperature oxidation characteristics of formaldehyde (HCHO) upon application of the catalysts. We found that the major factors determining the catalyst activity during catalyst preparation were the temperature used in liquid-phase reduction, the Pt/N2H4 ratio, and pH, whereas the reduction time and the addition of alkali metal had no significant effect. In addition, compared to the HCHO oxidation performance of catalysts obtained via the general method employed (gas-phase reduction), that of the catalysts obtained via liquid-phase reduction were superior. This result indicates that H2O significantly contributes to enhancing the catalytic activity when using liquid-phase reduction method-based catalysts as the conversion of HCHO into reaction intermediates is accelerated and the hydroxyl groups serving as adsorption sites are restored.