The recombination of allyl radicals (C3H5), generated from the dissociation of 1,5-hexadiene or allyl iodide dilute in krypton, has been investigated in a diaphragmless shock tube using laser schlieren densitometry, LS, (900-1700 K, 10 +/- 1, 29 +/- 3, 57 +/- 3, and 120 +/- 4 Torr). The LS density gradient profiles were simulated and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C3H5I -> C3H5 + I and C3H5 + C3H5 -> C6H10 were obtained and showed strong fall-off. Second order rate coefficients for allyl radical recombination were determined as k(1a,124Torr) = (2.6 +/- 0.8) x 10(55) T-12.995 exp(-8426/T), k(1a,57Torr) = (1.7 +/- 0.5) x 10(60) T-14.49 exp(-9344/T), and k(1a,30Torr) = (7.5 + 2.3) x 10(66) T-15.935 exp(-10192/T) cm(3) mol(-1)s(-1). The contribution of a disproportionation channel in allyl radical reactions was assessed, and the best agreement was obtained with no more than 5% disproportionation. Notably, because both the forward and back reactions of C(6)H(1)0 reversible arrow C3H5 + C3H5 were measured, utilizing two different precursors, the equilibrium constant of this reaction could be found, suggesting an entropy of formation of 1,5-hexadiene, 87.3 cal mol(-1) K-1, which is significantly smaller than that group additivity predicts, but larger than other reference literature values.