Time-resolved studies of the title reactions have been carried out over the pressure range 1-100 Torr (in SF6 bath gas) and at temperatures in the range 293-600 K, using laser flash photolysis techniques to generate and monitor the silylenes, SiH2 and SiMe2. All three reactions showed evidence of pressure dependence, consistent with third-body assisted association reactions to form silirane products. Extrapolation of the pressure-dependent rate constants gave the following Arrhenius parameters : SiH2 + C3H6, log(A/cm(3) molecule(-1) s(-1)) = -9.79 +/- 0.03, E-a (kJ mol(-1)) = -1.9 +/- : 0.3; SiH2 + C4H8, log(A/cm(3) molecule(-1) s(-1)) = -9.91 +/- 0.04, E-a (kJ mol(-1)) = -2.5 +/- 0.3; SiMe2 + C4H8, log(A/cm(3) molecule(-1) s(-1)) = -12.12 +/- 0.02, E-a (kJ mol(-1)) -8.5 +/- 0.2. These parameters are consistent with fast, nearly collision-controlled processes for SiH2 but a tighter transition state for SiMe2. Rice, Ramsperger, Kassel, Marcus theory (RRKM) modeling, based on consistent transition states for silirane decomposition, and employing a weak collisional deactivation model, gave good fits to the pressure-dependent curves for each system, provided an appropriate value of E-a (fitting parameter) was used for each reaction. The kinetic results are consistent with an electrophilically led addition mechanism, although methyl substitution in the alkene hardly affects the rate constants. The RRKM-derived E-o values have been used to derive reaction enthalpies which are in reasonable agreement with values obtained by ab initio calculations at the G2 (MP2,SVP)level. The experimental Delta H degrees values yield strain energies of 190, 196, and 216 kJ mol(-1) for 2-methyl-, 2,2-dimethyl-, and I,1-dimethylsilirane, respectively. Compared to the strain enthalpy of 167 kJ mol(-1) for silirane itself, this shows that methyl substituents in the silirane products substantially increase the strain energies. Theory supports this.