Rate coefficients for the reaction C2H+C2H2→C4H2+H are measured at 90 and 120 K by using a new pulsed Laval nozzle apparatus equipped with laser ionization, time-of-flight mass spectrometric detection. The C2H radicals are generated by 193 nm laser photolysis of C2H2, and the reaction product, C4H2, is directly detected by single-photon ionization at 118 nm (10.5 eV). Rate coefficients of (2.7¿0.5)¿10-10 and (2.5¿0.4)¿10-10 cm3 molecule-1 s-1 are obtained at 90 and 120 K, compared to the room temperature value of 1.3¿10-10 cm3 molecule-1 s-1. The results show a significant negative temperature dependence of the reaction rate coefficient below room temperature. The implications for models of planetary atmospheres, such as for Titan, are discussed. ¿ 2000 American Geophysical Union