The microwave spectra of four isotopic species of 3-methyl-1-butyne, isopropylacetylene, (CH 3) 2CHCCH, have been analyzed. With certain assumed structural parameters the following parameters were calculated: r (CC) = 1.495 ± 0.011 Å, r (CC) = 1.527 ± 0.003 Å, ∢ (CCC) = 109.6±0.2° and ∢(CCC) = 112.9±0.3°. The dipole moment was determined to be 0.721 ± 0.009D. A tentative assignment of the vibrational excited states indicates that the barrier to internal rotation of the methyl groups is 3650 ± 500 cal/mole. The results are compared to those of similar molecules, especially with cyclopropylacetylene.