Recent efforts to relate soil aeration to crop production rely in part on soil oxygen diffusion rate (ODR) measurements. The platinum microelectrode method is often used to measure the soil ODR. The electrode current is easily interpreted when the soil is saturated with water. When the soil is not saturated, the interpretation of the electrode current depends on the effective potential applied to the microelectrode. The effective potential cannot be measured directly, but is calculated from several other measured quantities. The equation for obtaining the effective potential is given for two types of ODR measurement systems. The second type is commercially available and offers several advantages, but has limited accuracy at low ODRs. Two modifications to the commercial system are described, both of which greatly increase the accuracy of the system.