The allyl-methyl and propargyl-methyl cross-radical reactions were studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 301-800 K and bath gas (helium) density (3-36) x 10(16) atom cm(-3). The observed overall C3H5 + CH3 (1) and C3H3 + CH3 (2) rate constants demonstrate negative temperature dependencies. Master equation modeling of collisional effects indicates that the C3H5 + CH3 reaction is near its high-pressure limit under all experimental conditions used. Minor corrections for the falloff effects ton average, 18% at the highest temperature) applied to the experimental values result in the high-pressure-limit temperature dependence of the rate constant of reaction 1: k(1)(infinity) = 1.55 x 10(-9) T-0.54 exp(117 K/T) cm(3) molecule(-1) s(-1). The temperature dependence of k(1)(infinity) combined with the thermochemistry of reaction 1 results in the rate constants of the reverse reaction of thermal decomposition of 1-C4H8 k(-1)(infinity)(T) 1.1 x 10(16)exp(-39100 K/T) s(-1). On the other hand the C3H3 + CH3 reaction is not near the high-pressure limit and falloff in reaction 2 cannot be neglected at temperatures above 500 K. Falloff corrections applied to reaction 2, on average, reach a factor of 2 at &00 K and introduce substantial uncertainties in the extrapolated high-pressure-limit rate constant values. The evaluated high-pressure-limit rate constants of reaction 2 can be represented with the expression k(2)(infinity) = 6.80 x 10(-11) exp(130K/T) cm(3) molecule(-1) s(-1). C4H8 and C4H6 were identified as primary products of the reactions 1 and 2, respectively.