The structure, dynamics, and thermal reactions of the radical cations of silacyclohexanes (cSiC5) containing one Si atom in the six-membered ring have been studied by means of ESR spectroscopy and ab-initio MO calculations. The cSiC5 radical cations were generated in perfluoromethylcyclohexane matrix by ionization radiation at low temperature. By the help of selectively deuterated or methylated silacyclohexanes the 4.2 K ESR spectrum of cSiC5(+) was successfully analyzed in terms of four different isotropic H-1 hyperfine coupling (hfc) constants: 7.55 mT to the equatorial hydrogen (H-3e) at the C(3) position, 3.45 mT to H-6e, 2.85 mT to H-5e, and 2.60 mT to H-2e. The result strongly suggests that the cSiC5 radical cation takes an asymmetrically distorted C-1 structure with one of the Si-C bonds elongated in which the unpaired electron mainly resides. Ab-initio MO calculations support the distorted structure. A suggested mechanism for the structural distortion is a pseudo-Jahn-Teller effect. Temperature dependent ESR line shapes were observed between 4.2 and 130 K. They were analyzed in terms of the dynamics of two-site jumping between two equivalent and asymmetrically distorted mirror image structures so as to give a selective bond length alternation between two adjacent Si-C bonds. The jumping rate was evaluated to be 12 MHz at a low temperature of 4.2 K and independent of temperatures below 40 K to give a nonlinear Arrhenius plot. The dynamics is suggested to be caused by a quantum mechanical tunneling effect at zero-vibrational levels of a symmetrical double-well potential surface. Furthermore, thermal reactions of cSiC5(+) are presented.