The rotationally resolved A 2 Π u <-X 2 Π g electronic origin band spectrum of tetraacetylene cation (HC 8 H + ) and its deuterated derivative (DC 8 D + ) has been recorded in the gas phase, applying both frequency modulation spectroscopy in a liquid nitrogen cooled hollow cathode discharge and cavity ringdown spectroscopy in a supersonic planar plasma. The combined analysis of the rotational structure yields accurate molecular constants for the ground and electronically excited states as well as information on the molecular geometry.