A novel and sensitive spectrophotometric method for the determination of molybdenum at trace levels in environmental and biological samples is proposed. The method is based on the reaction of Mo (V) with thiocyanate (SCN-) and methyltrioctyl ammonium chloride (MTOAC) in acidic medium. The red colored complex of molybdenum is extracted with N-phenylbenzimidoyl thiourea (PBITU) in 1-pentanol for its determination by spectrophotometry. The sensitivity of the present method is higher than other conventional thiocyanate method, due to the use of MTOAC in liquid-liquid extraction. The value of molar absorptivity of the complex with respect to molybdenum is 7.6 x 10(4) Lmol(-1) cm(-1) at 470 nm. The limit of detection of the metal is 5 ng mL(-1). The system obeys Beer's law between 20 and 1000 ng mL-1 with slope, intercept and correlation coefficient values of 0.81, 2.5 x 10(-3) and +0.999, respectively. Most of the metal ions tested did nor interfere in the determination of molybdenum. The proposed method has been Successfully applied for the determination of the rnolybdenum in environmental and biological samples. (C) 2008 Elsevier B.V. All rights reserved.