A combined matrix and gas phase study is presented to identify the A (3)Sigma (-)-X (3)Sigma (-) electronic transition of the linear triplet isomer of HC6N and isotopic derivative DC6N. Absorption spectra have been observed in a 6 K neon matrix after mass selective deposition and in the gas phase by cavity ring down spectroscopy through a supersonic planar plasma. The band origin of the 0(0)(0) A (3)Sigma (-)-X (3)Sigma (-) electronic transition of HC6N is determined to be at 21 208.60(5) cm(-1), shifted similar to 30 cm(-1) to the blue of the neon matrix value. Rotational analysis indicates that the chain is slightly stretched on electronic excitation, yielding B-0'=0.027 92(5) cm(-1). Transitions to vibrationally excited levels in the upper A (3)Sigma (-) state are observed as well. The results are compared with a rotationally resolved spectrum of the 0(0)(0) A (3)Sigma (-)(u)-X (3)Sigma (-)(g) electronic transition of the isoelectronic HC7H species.