The phosphinidene complex Ph-P-W(CO)(5) reacts with cyclopentadiene, 1,3-cyclohexadiene, 1,3-cycloheptadiene, and 1,3-cyclooctadiene to give in each case isomeric mixtures of syn- and anti-vinylphosphiranes. With longer reaction times and/or higher reaction temperatures, the anti adducts isomerize to the syn adducts. These epimerizations are likely to proceed via biradical intermediates. Only the syn-vinylphosphirane of 1,3-cyclohexadiene undergoes a [1,3]-sigmatropic shift to yield a syn-phopholene. The dichotomy of biradical and concerted mechanisms is discussed in relationship with the analogous mechanism for the vinylcyclopropane --> cyclopentene rearrangement.