The UV photolysis of n-pentanal in the 280-330-nm region has been studied in 5-nm intervals by using dye laser photolysis in combination with cavity ring-down spectroscopy. Absorption cross sections of n-pentanal were measured at each wavelength studied. n-Pentanal exhibited a broad, structureless absorption band similar, in appearance, to that of previously studied short-chain aldehydes. The absorption spectrum peaked at 295 nm with a cross section of (6.56 +/- 0.17) x 10(-20) cm(2) molecule(-1). The formation of the HCO radical, which is a photodissociation product, was monitored in these experiments. The HCO yield was found to be independent of n-pentanal pressure (2-18 Torr) and total pressure (8-480 Torr) except for 325- and 330-nm photolysis where the size of the HCO signal was small and the dissociation was near the threshold. The dependence of the HCO radical yield on the photolysis wavelength was determined. The HCO yields were 0.058 +/- 0.006, 0.095 +/- 0.009, 0.10 +/- 0.02, 0.14 +/- 0.01, 0.10 +/- 0.02, 0.15 +/- 0.02, 0.14 +/- 0.02, 0.20 +/- 0.06, 0.14 +/- 0.02, 0.085 +/- 0.034, 0.087 +/- 0.015 at 280, 285, 290, 295, 300, 305, 310, 315, 320, 325, and 330 nm, respectively, where uncertainty reflects experimental scatter only. End products from closed-cell photolysis of n-pentanal with and without O-2 were investigated at 290, 310, and 330 nm by using GC/MS and HPLC. Acetaldehyde was found to be a significant product from the photodissociation of n-pentanal/N-2 mixtures. Photolysis rates of n-pentanal to form HCO were calculated for two representative atmospheric conditions (noontime at sea level and 40 degrees N latitude on January 1 and on July 1). The estimated radical formation rate constants from n-pentanal photolysis were about twice as fast as those obtained from acetaldehyde photolysis.