A direct comparison of Josephson Voltage Standards (JVS) at 10 V between the Bureau International des Poids et Mesures (BIPM) and the Centro Nacional de Metrología (CENAM), México, was carried out in September 2011. This comparison is part of the BIPM key comparisons (BIPM.EM-K10.b), and took place in the new DC voltage laboratory of CENAM. The relative voltage difference between the two quantum standards was −6 parts in 1011 (−0.6 nV), with a relative Type A uncertainty of 4.5 parts in 1011 (0.45 nV). The total combined uncertainty of the comparison is under evaluation.