We present a simple fiber-optics probe system that could be used in any lab for convenient determination of optical properties of liquid phantoms based on diffuse reflectance and transmittance measurements in the visible/near-infrared region. We employed Monte Carlo simulations to determine the optimal system setup and to test the inverse algorithm employed to extract the optical properties from measured reflectance and transmittance. The inverse algorithm involved obtaining the fit merit function for values within the optical property range and determining the minimum. The performance of the method was tested by predictive error and validated using similar matrix of milk–ink phantoms on reflectance and transmittance. In the range of optical properties of phantoms with optical properties of 0 to 0.5  cm^−  1 for μ_a and 20 to 140  cm^−  1 for μ_s, the median prediction error for the test phantoms at 630 nm was 1.51% for μ_s and 8.82% for μ_a. The median difference in predicted values versus expected values was 1.15  cm^−  1 for μ_s and 0.01  cm^−  1 for μ_a. In comparison with other techniques, our method was a simple, fast, and convenient way to determine optical properties of liquid phantoms.