This study was about the release of free formaldehyde from tetrakis (hydroxymethyl) phosphonium (THP) salts and various syntan leather products, and the antioxidative blocking defense mechanisms of various essential oils. For that purpose, Citrus reticulata L., Melaleuca alternifolia, Origanum onites and Thymus vulgaris essential oils of 2% were used during fatliquoring. Formaldehyde analyses in finished leathers were performed according to TS EN ISO 17226-1 international standard by high performance liquid chromatography (HPLC) test method. To make a comparison on the analyses, some leather samples as a control group were processed without any essential oil. The results were evaluated statistically by using One-Way ANOVA and Duncan tests with the SPSS 15 statistic program. The results showed that Origanum onites essential oil used in fatliquoring process was significantly determined to eliminate the release of free formaldehyde in leather.