A two-layer model has been used to determine the mixing of C3H8 with the plasma stream in the decomposition of C3H, in an atmospheric argon arc plasma jet. The effect of argon flow rates and reactor diameters on the mixing is discussed in terms of @, the mass exchange coefficient between the hot core and low-temperature annular regions in the plasma stream. The model explains well the dependence of conversion on both the specific energy and the axial temperature change of the plasma at an argon flow rate of 8.6 L/min, by assuming that @ ranges from to 'Ilo for reactor diameters of 12 and 25 mm. At a high argon flow rate of 25.8 L/min, @ ranging from ' /*,, to also reproduces the experimental results for the reactor diameter of 25 mm. The range of p suggests that the mixing of C3H8 with plasma stream occurs stepwise.