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tokens: C of CH 2 and the C atoms can scramble, fractionation will occur. The dominant contribution of reaction ( 8 ) extends from somewhat greater than 10 3 yr to almost 10 5 yr, which is a much longer period than the time range when reaction ( 6 ) is dominant. Moreover, its range of dominance coincides more closely with the age range determined in multi-molecule fits to abundances in dark clouds (Wakelam et al. 2006 ). This reaction should, therefore, contribute to the differences in abundances between 13 CCH and C 13 CH, as mentioned before. Now consider the case of the isotopomer-exchange reaction:which is exothermic, as written from left to right so that the net effect is an increase in the abundance of the 12 C 13 CH isotopomer at the expense of 13 C 12 CH isotopomer. Furuya et al. ( 2011 ) used 1 × 10 −10 cm 3 s −1 as the rate coefficient of the forward reaction of the isotopomer-exchange reaction in their calculation. They also applied 8.1 K to the zero-point energy difference of 13 CCH and C 13 CH. The efficiency of this process depends upon a number of factors, including whether the exothermicity of 8.1 K is sufficient to cause the difference in observed abundance, whether the abundance of atomic hydrogen is large enough, and whether or not a barrier to reaction exists. The differences in abundances between 13 CCH and C 13 CH can be seen even at the early stages (Figure 3 in Furuya et al. 2011 ). This isotopomer-exchange reaction appears to be able to explain, at least partially, the 13 C isotopic fractionation in both L1521B and L134N. In summary, we found that C 13 CH is more abundant than 13 CCH in L1521B and L134N from our observations. This tendency agrees with the previous observations in TMC-1 and L1527 (Sakai et al. 2010 ). The higher abundance of C 13 CH compared with 13 CCH may indeed be common for dark clouds. Both reaction ( 8 ) and the isotopomer-exchange reaction likely contribute to the 13 C isotopic fractionation of CCH both in L1521B and L134N. 4.2. The Dilution of the <sup>13</sup>C Species in Dark Clouds 4.2.1. Comparisons of the <sup>12</sup>C/<sup>13</sup>C Ratios between CCH and HC<sub>3</sub>N among Dark Clouds It is interesting to compare the 12 C/ 13 C ratios of CCH and HC 3 N among the three dark clouds: L1521B, L134N, and TMC-1. Table 3 summarizes the 12 C/ 13 C ratios of CCH and HC 3 N in the three dark clouds and Figure 5 shows the comparisons. In the local ISM, the elemental 12 C/ 13 C ratio has been determined to be 60–70 (e.g., Milam et al. 2005 ), which we indicate as the yellow range in Figure 5 . Figure 5. Comparisons of the 12 C/ 13 C ratios of CCH and HC 3 N in the three starless cores; L1521B, L134N, and TMC-1. The errors indicate the standard deviation. The yellow range ( 12 C/ 13 C = 60–70) shows the values in the local interstellar medium. The values are the same as Table 3 . Table 3 The 12 C/ 13 C Ratios of CCH and HC 3 N in Dark Clouds Notes. a Taken from Sakai et al. ( 2010 ). b Taken from Taniguchi