High resolution fluorescence, one-photon excitation and two-photon excitation spectra of cis- and trans-1,3,5,7,9,-decapentaene have been measured in n-alkane matrices at 4.2 K and vibronic progressions have been analyzed. Trans-decapentaene in decane occupies a centrosymmetric site and one-photon transition occurs via mixing with a low frequency promoting vibrational mode. Cis-decapentaene can occupy two different sites in the crystal lattice of heptane with comparable probabilities. Some vibrations associated with the cis-double bond are observed in the emission spectra of cis-decapentaene. The totally symmetric double bond stretch vibration has a higher frequency in the excited 2 (1)A(g) State (approximate to 1790 cm(-1)) than in the ground 1 (1)A(g) state (approximate to 1600 cm(-1)). The appearance of another double bond vibrational band around 1530 cm(-1) in the excitation spectra of trans-decapentaene may be evidence of substantial mixing of normal vibrational modes in the first excited 2 (1)A(g) electronic state. The two-photon excitation spectra of the 1 (1)A(g) -->2 (1)A(g) electronic transition of trans-decapentaene in n-decane and n-heptane exhibit vibronic progressions based on overtones of the totally symmetric double bond stretch vibrational mode. Five overtones of this mode are observed in the region 24 700-33 000 cm(-1) and the vibronic progression appears to be nearly harmonic. No other A, electronic states with an appreciable two-photon cross section have been observed in the region below 40000cm(-1).