Measuring optical properties of onions is important for evaluating the quality of onions using optical or spectral techniques. This paper reports the measurements of the absorption and scattering properties of single-layer onion tissues at 632.8 nm. The outermost dry skin, 1st layer flesh scale, and 2nd layer flesh scale of "Century" and "Vaquera" onions were tested. An integrating sphere system was used to measure the total diffuse reflectance and total transmittance of the sandwiched onion tissues. The absorption and reduced scattering coefficients of the onion tissues were determined using the inverse adding-doubling method based on the measured reflectance and transmittance values. The light penetration depths in onion tissues were also estimated. The results showed that the mean absorption and reduced scattering coefficients of onion dry skin were about 40-50 times higher than the values of onion flesh tissues. The absorption and scattering coefficients of onion dry skin showed significant differences between "Century" and "Vaquera" onions. Also, the reduced scattering coefficients were much higher than the absorption coefficients for both onion dry skin and onion flesh tissue. The mean estimated light penetration depths in the dry skins of Vaquera and Century onions were 0.83 ± 0.55 mm and 2.61 ± 2.92 mm, respectively. The mean estimated light penetration depths in the flesh scales of Vaquera and Century onions was about 35.72 ± 16.08 mm. The results presented in this study can be used for developing appropriate optical sensing approaches to test the quality attributes of onions.