A simple high performance liquid chromatographic (HPLC) method and a highly sensitive gas chromatography mass spectrometric (GC-MS) method have been established for the determination of ortho-phthalaldehyde (OPA) in water. These methods are based on the derivatization of OPA with hydrazine in water. The following optimum reaction conditions were established: reagent dosage, 20 mg/mL of hydrazine; pH 2; reaction for 20 min at 70 °C. The organic derivative was detected directly by HPLC or after the extraction with methylene chloride/concentration by GC-MS. The limit of detection of OPA in water was 4.0 and 0.3 μg/L by HPLC and GC-MS, respectively. The calibration curve showed good linearity with r² = 0.9993 and r² = 0.9994 by HPLC and GC-MS, respectively, the accuracy was in a range of 95-105%, and the precision of the assay was less than 13% in water. The HPLC method was simple and reproducible enough to permit the OPA content analysis in the disinfectant products, and the GC-MS method is sensitive enough to permit reliable analysis of OPA to the μg/L level in environmental water.