AbstractThree isotopically enriched samples of cyclobutene have been prepared, namely, a mixture containing 20 % [1‐13C]cyclobutene, 20 % [3‐13C]cyclobutene and 60 % parent molecule; a second mixture of 75 % [1‐D]cyclobutene and 25 % parent molecule; and a third mixture of 50 % [3‐D]cyclobutene and 50 % parent molecule. Proton magnetic resonance and infrared spectra showed the samples to be of high purity (>95 %). The preparations successfully have been used for high‐resolution microwave studies. The observed microwave line intensities also agreed with the percentage compositions gi