We report the direct measurement of the photolysis rate coefficients of ethyl, n-propyl, n-butyl, and 2-butyl nitrate to NO2 and the alkoxy radical, RONO2+hv→RO+NO2, using a luminol chemiluminescence NO2 detector. The photolysis of primary and secondary alkyl nitrates is slow: at a temperature of 288 K, a zenith angle of 60¿, and an ozone column abundance of 0.295 atm cm, jRONO2≈0.40¿10-6 s-1 for the primary alkyl nitrates, and 0.65¿10-6 s-1 for 2-butyl nitrate. This implies a photolytic lifetime of 15 to 30 days in the planetary boundary layer. The photolysis frequency is very temperature dependent, with an effective activation energy of 11.2 kJ mol-1 for 2-butyl nitrate and 10.1 kJ mol-1 for ethyl nitrate. Observed photolysis rate coefficients agree, with experimental uncertainty, with coefficients calculated using recently measured absorption cross sections and an assumed quantum yield of unity. The photolysis rate coefficients of ethyl, n-propyl, and 2-butyl nitrate for an overhead Sun, a temperature of 298 K, and an ozone column abundance of 0.295 atm cm are estimated to be 1.45, 1.33, and 2.54¿10-6 s-1, respectively. Henry's law constants (H) at a temperature of ~295 K are 1.26¿0.12 M atm-1 and 0.74¿0.04 M atm-1 for n-butyl and 2-butyl nitrate, respectively. At a temperature of ~279 K, H increases to 3.94¿0.59 M atm-1 for n-butyl nitrate and 2.27¿0.23 M atm-1 for 2-butyl nitrate. Wet deposition is concluded to be an unimportant sink for alkyl nitrates in the absence of aqueous phase reactions. ¿ American Geophysical Union 1989