Rate constants for the gas-phase reactions of NO3 radicals and O-3 with a series of C-6-C-14 1-alkenes and 2-methyl-1-alkenes have been measured at 296 +/- 2 K and atmospheric pressure of air using relative rate methods. For the NO3 radical reactions, the rate constants obtained (in units of 10(-14) cm(3) molecule(-1) s(-1)) were: 1-hexene, 2.00 +/- 0.16; 1-octene, 2.35 +/- 0.15; 1-decene, 2.55 +/- 0.16; 1-dodecene, 2.79 +/- 0.36; 1-tetradecene, 2.87 +/- 0.21; 2-methyl-1-pentene, 43.8 +/- 2.3; 2-methyl-1-hexene, 52.4 +/- 2.5: 2-methyl-1-octene, 57.8 +/- 2.6; 2-methyl-1-nonene. 60.8 +/- 2.9; 2-methyl-1-undecene, 60.8 +/- 3.3: 22-methyl-1-tridecene, 60.3 +/- 3.4; and cycloheptene, 49.4 +/- 2.0. For the O-3 reactions, the rate constants obtained (in units of 10(-17) cm(3) molecule(-1) s(-1)) were: 1-hexene, 0.898 +/- 0.054; 1-heptene, 1.05 +/- 0.07: 1-octene, 1.01 +/-0.04; 1-decene, 1.11 +/- 0.05; 1-dodecene, 1.38 +/- 0.14; 1-tridecene. 1.92 +/- 0.12; 1-tetradecene, 2.44 +/- 0.24; 2-methyl-1-pentene, 1.26 +/- 0.13; 2-methyl-1-heptene, 1.35 +/- 0.05: 2-methyl-1-octene, 1.38 +/- 0.06; 2-methyl-1-decene, 1.48 +/- 0.07; 2-methyl-1-undecene. 1.46 +/- 0.11; and 2-methyl-1-tridecene, 2.85 +/- 0.42. The rate constants for the NO3 radical reactions significantly increase with increasin-carbon number, attaining a plateau at >= C-14 for the 1-alkenes and at C-10-C-14 for the 2-methyl-1-alkenes. In contrast, the rate constants for O-3 relations increase only slightly with increasing cat-boil number up to similar to C-10 for the 1-alkenes and similar to C-12 for the 2-methyl-1-alkenes. with the significant increase ill the measured rate constants for the >C-10 1-alkenes and >C-12 2-methyl-1-alkenes possibly being due to heterogeneous reactions. Reasons for the observed trends in NO3 radical and O-3 reaction rate constants with alkene carbon number are discussed.