The origin band of the A(2)Pi(u) - X(2)Pi(g) electronic transition of the linear pentaacetelyne cation, HC10H+, and isotopic derivatives, HC10D+ and DC10D+, has been recorded in the gas phase. The absorption spectrum was observed by cavity ring down spectroscopy through a supersonic planar plasma expansion. The spectrum comprises both spin-orbit components with resolved P- and R-branches. Contour fits allow the determination of ground and excited state spectroscopic constants.