The thermal dissociation of SO3 has been studied for the first time in the 1000-1400 K range. The experiments were conducted in a laminar flow reactor at atmospheric pressure, with nitrogen as the bath gas. On the basis of the flow reactor data, a rate constant for SO3 + N-2 -> SO2 + O + N-2 (R1b) of 5.7 x 10(17) exp(-40000/T) cm(3)/(mol s) is derived for the temperature range 1273-1348 K. The estimated uncertainty is a factor of 2. The rate constant corresponds to a value of the reverse reaction of k(1) approximate to 1.8 x 10(15) cm(6) mol(-2) s(-1). The reaction is in the falloff region under the investigated conditions. The temperature and pressure dependence of SO2 + O (+ N-2) was estimated from the extrapolation of low temperature results for the reaction, together with an estimated broadening parameter and the high-pressure limit determined recently by Naidoo, Goumri, and Marshall (Proc. Combust. Inst. 2005, 30, 1219-1225). The theoretical rate constant is in good agreement with the experimental results. The improved accuracy in k(1) allows a reassessment of the rate constant for SO3 + O -> SO2 + O-2 (R2) based on the data of Smith, Tseregounis, and Wang ( Int. J. Chem. Kinet. 1982, 14, 679-697), who conducted experiments on a low-pressure CO/O-2/Ar flame doped with SO2. At the location in the flame where the net SO3 formation rate is zero, k(2) = k(1)[SO2][M]/[SO3]. A value of 6.9 x 10(10) cm(3) mol(-1) s(-1) is obtained for k(2) at 1269 K with an uncertainty a factor of 3. A recommended rate constant k(2) = 7.8 x 10(11) exp(-3065/ T) cm(3) mol(-1) s(-1) is consistent with other flame results as well as the present flow reactor data.