The microwave spectra of benzocyclobutene, benzocyclobutene- (\alpha-d_{1}), and benzocyclobutene(\alpha, \alpha-d_{2}), generated by pyrolysis of 3-isochromanone, 2 -methylbenzyl chloride- (\alpha-d_{1}), and 2 -methylbenzyl chloride- (\alpha, \alpha-d_{2}), have been observed in the frequency range from 8.0 to (40.0 \mathrm{GHz}). The rotational constants (MHz) have been determined as (A=4442.38(4), B=2083.490(3)), and (C) (=1444.563(3)) for normal species, (A=4356.76(7), B=2026.916(5)), and (C=1414.307(8)) for monodeuterated species, and (A=4277.91(8), B=1972.942(7)), and (C=1386.022(9)) for dideuterated species in their ground vibrational states. The two excited vibrational states assigned in normal species are of the out-of-plane vibrational modes ( (v_{a}=1) and (\left.v_{b}=1\right)). The values of (\Delta I\left(=I_{c}-I_{a}-I_{b}\right)) for normal, monodeuterated, and dideuterated species were found to be (-6.478(2),-8.000(5)), and (-9.666(5) \mathrm{u} \AA^{2}), respectively. The (\Delta I) values suggest that only four hydrogen atoms are located out of the molecular skeletal plane of benzocyclobutene. The dipole moment ( (\mu_{a}) ) was determined to be (0.56(5)) D. C1 1993 Academic Press, Inc.