microwave spectrum of the Fe-56 and the Fe-54-isotopomer of tricarbonyl(isoprene) iron was recorded in the range 3-23 GHz using a Fourier transform microwave spectrometer working on a molecular beam. In all, 326 frequencies were measured for the Fe-56-isotopomer, where some transitions revealed a splitting due to the internal rotation of the methyl group in the diene ligand. Fitting these frequencies to the calculated spectrum yielded rotational constants, centrifugal distortion constants, and a barrier for the internal rotation with high accuracy. For the Fe-54-isotopomer, 23 transitions could be measured giving another three rotational constants. Here, the centrifugal distortion constants were constrained to the values obtained for the main isotopomer. The six rotational constants can be used to determine the most uncertain structural parameters starting from a geometry based on the structure of the tricarbonyl(butadiene-1,3) iron.