An investigation of the absolute rate constants of the metathesis reactions i-C3H + HI → C3H8 + I• (1) and n-C3H + HI → C3H8 + I• (2) was performed using a newly developed apparatus. The obtained Arrhenius expressions are k1 = 1.94 (±0.42) × 10−11 exp(−6.190 (±0.855)/RT) and k2 = 6.49 (±1.86) × 10−11 exp(−11.084 (±0.913)/RT) (R = 8.314 J K−1 mol−1) over the temperature range 293–623 K (A in cm3 molec−1 s−1, Ea in kJ mol−1). Two Knudsen reactors of different geometry coupled to single-photon (VUV) photoionization mass spectrometry (SPIMS) were successively used. The two propyl isomers were generated externally to and upstream of the reactor and led to thermalized i- and n-C3H free radicals. A minor correction to k2 for the wall loss of n-C3H (kw) was applied throughout the temperature range, whereas no significant wall loss of i-C3H could be measured. The obtained results are based on the disappearance of propyl radicals with increasing amounts of HI. Thermochemical parameters of propyl free radicals resulting from the present kinetic measurements are discussed and point toward a slightly lower value for the standard heat of formation of Δf(i-C3H) compared to the currently accepted value, whereas no significant adjustment for n-C3H seems to be necessary. The recommended values are Δf(i-C3H) = 86.6 ± 2.1 kJ mol−1 and Δf(n-C3H) = 101.1 ± 2.1 kJ mol−1 resulting from a third law evaluation using (i-C3H) = 289.9 and (n-C3H) = 290.3 J K−1 mol−1, both values being very close to the values measured by W. Tsang, namely 88 ± 2 and 100 ± 2 kJ mol−1, respectively.