The decomposition rates of isobutane and n-butane in the falloff regime at high temperatures were studied in a shock tube using UV narrow-line laser absorption of CH3 at 216.6 nm. Experimental conditions ranged from 1297 to 1601 K and 0.20 to 8.8 atm with mixtures varying in concentration from 198 to 400 ppm of isobutane or n-butane diluted in argon. Decomposition rate coefficients were determined by monitoring the formation rate of CH3 immediately behind shock waves and modeling the CH3 formation with detailed kinetic mechanisms. Calculations were performed using RRKM/master equation analysis with a restricted (hindered) Gorin model for the transition state and fit to the experimental data. The rate coefficient for isobutane decomposition, i-C4H10 --> CH3 + i-C3H7, from 1320 to 1560 K can be described, using the Troe pressure-broadening formulation, by k(infinity,1)(T) = 4.83 x 10(16) exp(-402 10 K/T) s(-1), k(0,1)(T) = 2.41 x 10(19) exp(-264 60 K/T) cm(3) mol(-1) s(-1), and F-cent,F-1(T) = 0.75 exp(-T/750 K). n-Butane decomposes via two reaction routes. The rate coefficient for n-C4H10 --> CH3 + n-C3H7 from 1320 to 1600 K can be described by k(infinity,2)(T) = 4.28 x 10(14) exp(-351 80 K/T) s(-1), k(0,2)(T) = 5.34 x 10(17) exp (-216 20 K/T) cm(3) mol(-1) s(-1), and F-cent,F-2(T) = 0.28 exp(-T/1500 K). And the rate coefficient for n-C4H10 --> C2H5 + C2H5 from 1320 to 1600 K can be described by k(infinity,3)(T) = 2.72 x 10(15) exp(-380 50 K/T) s(-1), k(0,3)(T) = 4.72 x 10(18) exp(-249 50 K/T) cm(3) mol(-1) s(-1), and F-cent,F-3(T) = 0.28 exp(-T/1500 K).