We have investigated one of the strongest superacid systems, SbF5 in liquid HF, by ab initio molecular dynamics simulation. In dilute solution a barrierless, diffusion-limited fluorination reaction takes place to form the SbF6- anion and H2F+ cation. The initial contact ion pair evolves to become a fully separated ion pair by a series of stepwise, incoherent proton jumps. On average, the SbF6- anion had an octahedral structure with the average bond length of 1.9 Angstrom. The cationic species is a protonated HF chain, which mediates the fast proton jump process.