Abstract: Spectrophotometry with 4-aminoantipyrine as reagent was applied to the determination of low contents of volatile phenol in surface water by using a waveguide fiber colorimetric cell with a long optical path of 100 cm. After pre-distillation of the sample, 25 mL of the distillate were taken, and 0.25 mL of buffer solution of NH₄Cl and aq. ammonia (pH 10.7), 0.5 mL of 2 g·L^-1 4-aminoantipyrine solution, and 0.5 mL of 80 g·L^-1 K₃Fe(CN)₆ solution were added in succession. After mixing, the solution was transferred to the long optical path colorimetric cell and the absorbance was measured at 510 nm. A linear relationship was found between the absorbance and the mass concentration of volatile phenol in the range of 1.00-40.0 μg·L^-1, with detection limit (3s) of 0.257 μg·L^-1. Recovery rates obtained by standard addition method were in the range of 95.7%-107%.