In chemistry, bond stretch isomerism is a concept of isomerism based on variations of bond length.  The concept was proposed in the 1970s but was refuted in the 1990s.
The phenomenon was first invoked to explain the observation of blue and green isomers of , where  is dimethylphenylphosphine.  These isomers were shown, purportedly, by X-ray crystallography to differ with respect to the length of the Mo-O bond, which differed by 0.2 Å.  Subsequent work showed that the supposed green bond stretch isomer consisted of blue  contaminated with a small amount of yellow .  The nearly isomorphous replacement of Mo-O unit with small amounts of Mo-Cl unit results in artifactually long Mo-O distance in the green sample. In essence the deception arises because the crystallographic disorder was not modeled appropriately.  Several such examples were uncovered.