Recently the rate constants of the reactions C 2 H 3 + H 2 → C 2 H 4 + H and C 2 H 3 + H → products have been measured by more direct methods than used previously. The new determination for the rate constant of C 2 H 3 + H 2 is lower by a factor of 1000 at room temperature than the rate used by Allenet al. in their methane photochemical model (The relative abundance of ethane to acetylene in the jovian stratosphere,Icarus100, 527–533 (1992)). A fast rate for this reaction is key to their proposed reaction scheme to convert C 2 H 2 to C 2 H 6 and explain the C 2 H 6 /C 2 H 2 ratio on Jupiter. However, their scheme becomes ineffective when a rate constant expression consistent with Fahret al. is used (Experimental determination of the rate constant for the reaction of C 2 H 3 with H 2 and implications for the partitioning of hydrogen in the atmospheres of the other planets,Icarus116, 415–422 (1995)). The rate measurement for C 2 H 3 + H by Monkset al. (Kinetics and products of the reaction between H and C 2 H 2 of T = 213 and 298 K,J. Phys. Chem.99, 17,151–17,159 (1993)) is consistent with recent measurements but they found C 2 H 4 to be a significant product, in addition to C 2 H 2 + H 2 . Only the latter product channel had been used in methane photochemical models. With the new lower rate constant for C 2 H 3 + H 2 , the reaction C 2 H 3 + H → C 2 H 4 becomes important in determining the C 2 H 6 /C 2 H 2 ratio on Jupiter. However, the new reaction scenario is less efficient in converting C 2 H 2 to C 2 H 6 than the one proposed by Allenet al. because it depends in part upon H, which is in scarce supply in the lower stratosphere where C 2 H 3 + H → C 2 H 4 is important.