It is possible to compare Josephson-array voltage systems to parts in 10/sup 9/ via multi-Zener reference interchanges and ultra-low thermal-emf switching. These techniques were used to achieve a Type A uncertainty of 6 nV/V (k=2) at the 1.018 V level, and to establish that the agreement between the two systems fell within the estimated 4 nV/V Type B relative uncertainty (k=2). The method is useful in checking system operation under normal measurement configurations and programmed control. Data also indicated that thermal-emfs greater than 25 nV have fluctuations that are not perfectly cancelled by either of the two differing measurement algorithms.