A synthetic procedure yielding a mixture of Z- and E-1-propenyl isocyanide (CH(3)CH═CHNC) is described. The microwave spectrum of this mixture has been recorded in the 12-100 GHz spectral range, and the spectra of the Z and E isomers have been assigned for the first time. Most transitions of the Z form were split into two components of equal intensity due to tunneling of the methyl group, which allowed the barrier to internal rotation of this group to be determined as 4.0124(12) kJ/mol by fitting 568 transitions with a maximum value of J = 46 using the computer program Xiam. This fit had a root-mean-square deviation as large as 4.325. The same transitions were therefore fitted anew using the more sophisticated program Erham. This fit has a rms deviation marginally better (4.136) than the Xiam fit. No split MW lines were found for E-1-propenyl isocyanide. The absence of splittings is ascribed to a barrier to internal rotation of the methyl group that is significantly higher than the barrier of the Z isomer. It is concluded that the barrier must be larger than 6 kJ/mol for the E form. The experimental work was augmented by quantum chemical calculations at CCSD/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVTZ levels of theory. The CCSD method predicts rotational constants of the Z and E forms well. The B3LYP barriers to internal rotation of a series of substituted propenes were calculated and found to be in good agreement with experiments. Calculations of the quartic centrifugal distortion constants of the two 1-propenyl isocyanides by the B3LYP and MP2 methods were less successful.