The 1(1)Sigma(+)(u) <- 1(1)Sigma(+)(g) system in polyacetylenes HC2nH (n = 5-7) has been observed for the first time in the gas phase. The UV absorption spectra of these chains were recorded using the resonant. two-color two-photon ionization technique. A strong vibrational progression corresponding to the acetylenic stretching mode in the excited state has been observed. The absorption energy showed an inverse dependence on the chain length. The relaxation process of the 1(1)Sigma(+)(u) excited state has been discussed.