A reinvestigation of the absolute rate constant of the metathesis reactions t-C4H9 center dot + HBr -> i-C4H10 + Br-center dot (1) and t-C4H9 center dot + HI -> i-C4H10 + I-center dot (2) was performed thanks to a recently developed apparatus consisting of a Knudsen reactor coupled to detection based on single-photon (VUV) photoionization mass spectrometry (SPIMS). It enables the generation of thermalized hydrocarbon free radicals owing to a source upstream of and external to the Knudsen reactor. The following Arrhenius expressions were obtained: k(1) = 5.6(+/- 1.4) X 10(-12) exp(-6.76(+/- 0.94)/(RT)) and k(2) = 2.0(+/- 0.6) x 10(-11) exp(-8.48(+/- 0.94)/(RT)) with R = 8.314 J mol(-1) K-1 over the range 293 to 623 K. The mass balance of the reaction system based on closed shell product detection (CSPD) was checked in order to ensure the accuracy of the used reaction mechanism and as an independent check of k(1) and k(2). The wall-loss rate constants of the t-butyl free radical, kw(C4H9), were measured and found to be low compared with the corresponding escape rate constant, k(e)(C4H9), for effusion of t-C4H9 center dot out of the Knudsen reactor. On the basis of the present results, the free radical standard heat of formation Delta H-f(298)o(t-C4H9 center dot) = 44.3 +/- 1.7 kJ mol(-1) was obtained when combined with the kinetics of the inverse halogenation reaction taken from the literature and using S-298(o)(t-C4H9 center dot) = 322.2 J K-1 mol(-1) following a "Third Law" evaluation method. The standard enthalpy for t-butyl free radical is consistent for both the bromination and iodination reactions within the stated uncertainties.