We designed a redox apparatus that controls electropotenial (Eh), pH, and temperature. The apparatus is fully automated and can be assembled from equipment found in most soil analytical laboratories. The reactor can maintain environmental conditions used to evaluate the chemical fate of contaminants under natural conditions (natural attenuation) or adjust them to a new steady state to evaluate the effect of engineered remediation on a subsurface system. It can operate within a wide range of redox potential values (−400 to 400 mV) and at any pH value. The stability and accuracy of the instrument was tested by performing Eh-pH titrations on MnO2 suspensions. Results indicated that Eh could be controlled within 1 to 2 mV, pH within 0.05 to 0.1 unit, and temperature within 0.1°C for weeks or months.