ASME SA-213 T91 tubes and SA-335 P91 pipes were widely used in combined-cycle steam systems due to their greater increased temperature strength and creep behavior. In this paper, a practical and selective spectrophotometric method for the determination of molybdenum in ASME SA-213 T91 tube and SA-335 P91 pipe samples has been presented. This method was based on the chromogenic reaction between molybdenum(V) and sodium thiocyanate. Employing stannous chloride as a reductant, molybdenum(VI) was reduced to molybdenum(V) at the room temperature. The obtained molybdenum(V) formed an orange-red-colored (1:5) ligand complex with thiocyanate. The absorbance of the complex was measured at 470nm, and the molar extinction coefficient is 1.75´104L·mol-1·cm-1. Under the optimum reaction conditions the absorption value was proportional to the concentration of molybdenum in the range of 0.40~1.21%, and the relative standard deviation was less than 3.0%. The proposed method was free from the interference from a large number of analytical important elements and has been applied satisfactorily to the determination of molybdenum in grade T91 tube and P91 pipe samples with the improved accuracy and precision.