Cyclobutadiene is a notable exception in organic chemistry, partly because its ring has 4  electrons and thus does not follow Hückel's rule, although it has alternating single and double bonds. Numerous attempts have been reported to stabilize and isolate its structure and to measure its properties, most often in extreme temperature conditions. In this study we show how 1,3-dimethylcyclobutadiene can be prepared, isolated and confined in a calixarene bowl which is capped by guanidine molecules. The atomic structure of the crystalline matrix with and without 1,3-dimethylcyclobutadiene and several other derivatives have been determined by X-ray diffraction at low temperature.