As an alternative to agricultural adjuvant, the influence of aluminum oxide (Al2O3) on the detection of chlorpyrifos dried on apple skin using UV was investigated. UV spectral reflectance was measured in the range 240-400 nm from a mixture of chlorpyrifos and A1203 dried on apple skin. It was determined that chlorpyrifos reduced the reflectance at 290 nm, and Al2O3 raised the spectral reflectance in the UV test range. Reflectance at 380 nm was used for estimating the amount of Al2O3 dried on apple skin. The relationship between the depth of a valley appearing at approximately 290 nm in the UV spectral reflectance and the reflectance value at 380 nm was modeled using the saturation curve. This relationship was effective in constructing an accurate prediction model for chlorpyrifos amounts in the range 0.00491-0.0490 mg-cm-2. Reflectance data at 380 nm ranged from 0.051-0.41 for calibration. The proposed model could predict the amount of chlorpyrifos with a correlation coefficient of 0.84. Furthermore, the standard error of prediction is 8.40 × 10 -3 mg-cm-2. Conversely, the model was valid in the range 0.079-0.38 for reflectance at 380 nm.