Hydrogenated acrylonitrile butadiene rubber (HNBR) is a high-performance elastomer which has found important applications in the automobile and petroleum industries. Compared to current NBR solution hydrogenation technology, direct hydrogenation of NBR in its aqueous form (i.e. NBR latex) is a “green” strategy as it is energetically favorable and environmentally friendly. Two water-soluble rhodium based catalysts, RhCl(TPPMS)₃ (TPPMS = monosulfonated triphenylphosphane) and RhCl(TPPTS)₃ (TPPTS = trisulfonated triphenylphosphane), were investigated for NBR latex hydrogenation. The NBR in aqueous latex could be hydrogenated using the RhCl(TPPMS)₃ catalyst, attaining HNBR with as much as 95 mol% CC bond removal. A limited fraction of CC bond reduction was observed when RhCl(TPPTS)₃ was used as the catalyst. Using the RhCl(TPPMS)₃ catalyst, the hydrogenation reaction could be conducted at mild temperature with no addition of organic co-solvent. The presence of Rh metal in the resultant HNBR confirmed the phase transfer of the RhCl(TPPMS)₃ catalyst and was considered as a crucial factor for achieving a successful reaction. The observed influence of process conditions on the activity of hydrogenation has led to an extrapolation of a mild reaction condition mechanism for the RhCl(TPPMS)₃ system. Additionally, in-house NBR latices with different gel content were synthesized and investigated in the hydrogenation experiment for comparison. One of the major observations is that the high gel fraction in NBR could greatly limit the hydrogenation reaction which could be a reasonable explanation for the low conversion of CC bonds reported previously.