Electronic absorption and emission spectra have been investigated for cyanodiacetylene, HC(5)N, an astrophysically relevant molecule. The analysis of gas-phase absorption was assisted with the parallel rare gas matrix isolation experiments and with density functional theory (DFT) predictions concerning the excited electronic states. Mid-UV systems B (1)Delta<--X (1)Sigma(+) (origin at 282.5 nm) and A (1)Sigma(-)<--X (1)Sigma(+) (306.8 nm) were observed. Vibronic assignments have been facilitated by the discovery of the visible phosphorescence a (3)Sigma(+)<--X (1)Sigma(+) in solid Ar, Kr, and Xe. Phosphorescence excitation spectra, as well as UV absorption measurements in rare gas matrices, revealed the enhancement of A<--X transitions. The vibronic structure of dispersed phosphorescence spectra supplied new data concerning the ground state bending fundamentals of matrix-isolated HC(5)N. The experimental singlet-triplet splitting, 2.92 eV in Ar, closely matches the value of 3.0 eV predicted by DFT.