The rotational spectra of the HCCCNH+, NCCNH+, and CH3CNH+ ions have been observed in a supersonic molecular beam by Fourier transform microwave (FTM) spectroscopy. The rotational and centrifugal distortion constants were determined for all three, and the nitrogen quadrupole hyperfine coupling constants for HCCCNH+ and NCCNH+. From the respective Doppler shifts, it is found that the velocities of the ions are 3% larger than those of the parent unprotonated molecules, and the linewidths are greater by about 50%. The concentration of the ions near the nozzle is approximately 10(11) cm(-3), which is sufficiently high to be detectable in the visible and the IR by present laser techniques.