There are two allowed pathways for the thermal cheletropic decarbonylation of 3-cyclopentenone. The stereochemistry of decarbonylation of an unconstrained derivative (trans, trans-2,5-dimethyl-3-cyclopentenone, 4) has been determined for the first time. Under conventional pyrolysis conditions, thermal rearrangements of the initial product (trans, trans-2,4-hexadiene, 5) occur at the high temperatures required for the decarbonylation. However, by using multiphoton infrared photolysis/thermolysis to initiate decarbonylation, it was shown that the initial products from thermal decarbonylation of 4 are solely carbon monoxide and stereospecifically 5. The stereochemistry of decarbonylation is thus disrotatory, in accord with prior theoretical studies. A survey of crystal structures reveals ground-state distortions along this reaction coordinate as well.