Rotational spectra of HC4NC and HC6NC, linear molecules＇ of interest to interstellar cloud chemistry, were recorded by Fourier transform microwave spectroscopy, and the ground state rotational constants were determined to be 1401.18227(7) and 582.5203(1) MHz. Nitrogen quadrupole hyperfine structure could be observed for HC4NC. On the basis of coupled cluster calculations including connected triple substitutions accurate equilibrium structures (uncertainty in bond lengths ca. 0.0065 Angstrom) could be established for both species. The equilibrium dipole moments, : predicted to be -3-25 and -3.49 D for HC4NC and HC6NC, respectively, exhibit large correlation effects of 30% and 33%