P-31{H-1} NMR spectroscopy has been used to study the interconversion of trans-W2Cl4(NHCMe(3))(2)(PR(3))(2) and cis-W2Cl4(NHCMe(3))(2)(PR(3))(2) (R(3) = Me(3), Me(2)Ph). The coupling constants between W-183 and P-31 are about 110 and 300 Hz for the trans and cis isomers, respectively, which is consistent with the differences in W-P bond distances. The cis and trans isomers come to equilibrium in solution, with detectable amounts of both present. The [trans]/[cis] equilibrium constant at 50 degrees C for the PMe(2)Ph system is 0.40 +/- 0.05. The rates of the isomerization for the PMe(2)Ph system is greatly slowed down by free phosphine in solution, but they are basically unchanged when free phosphine concentrations increase from 0.0021 to 0.415 M. Competing mechanisms, one involving phosphine dissociation as the rate determining step and the other a unimolecular one are proposed. It is suggested that the unimolecular mechanism may be one involving internal flip processes and that the flip barrier has a value in the range of 25-30 kcal/mol(-1).