The diffusion mechanism of an excess proton in imidazole molecule chains is studied by Car-Parrinello-type ab initio molecular dynamics simulations. The diffusion process is described by a Grotthuss mechanism (structure diffusion) involving proton transfer and local rather than long-range cooperative reorientation of the imidazole chain. At T = 390 K, the proton transfer step is found to be fast with a time scale of 0.3 ps. The reorientation step is found to be rate-determining. According to our model, the time scale for the reorientation step is estimated to be approximately 30 ps in this temperature range.