Cyclobutadiene (CBD), the smallest cyclic hydrocarbon bearing conjugated double bonds, has long intrigued chemists on account of its strained geometry and electronic instability, but the parent compound and its unperturbed derivatives have thus far eluded crystallographic characterization. In this work, we immobilize a precursor, 4,6-dimethyl- -pyrone, in a guanidinium-sulfonate-calixarene (G4C) crystalline network that confines the guest through a combination of CH- and hydrogen-bond interactions. Ultraviolet irradiation of the crystals transforms the entrapped 4,6-dimethyl- -pyrone into a 4,6-dimethyl- -lactone Dewar intermediate that is sufficiently stable under the confined conditions at 175 kelvin to allow a conventional structure determination by x-ray diffraction. Further irradiation pushes the reaction to completion, enabling the structure determination of 1,3-dimethylcyclobutadiene Me2CBD. Our data support experimental observation of square-planar (Me2CBDS) and rectangular-bent (Me2CBDR) geometries in the G4C host matrix. The hydrogen-bonded, dissociated carbon dioxide coproduct interacts more strongly with Me2CBDS than with Me2CBDR.