Pyrolysis of two differently labeled versions of 3-phenylphthalic anhydride shows that a m-benzyne can form the related o-benzyne through shift of a phenyl group. The highest energy point in the process is the transition structure for a reverse carbon-hydrogen insertion in an intermediate benzopentalene. With the minor addition of an intermediate alkyne formed through a Roger Brown rearrangement, the original mechanism for formation of acenaphthalene accommodates the labeling results.