This article analyses the effect of researchers' postdoctoral mobility on academic performance. Postdoctoral positions are considered and conceptualised as a special type of researcher mobility. We use the curriculum vitae of UK academic scientists as a source of information, in addition to the ISI Web of Knowledge and the European Patent Office. We find major differences in the patterns of mobility between the pure and the transfer sciences. Pure scientists tend to move via a postdoctoral appointment, whereas transfer scientists tend to change job positions. We find that international postdoctoral mobility is positively correlated with publications for non job-mobile pure scientists in a five-year period starting two years after completion of the PhD, and with the total number of citations. The explanation is that weak institutional ties connected with postdoctoral fellowships give researchers access to institutions with higher reputation, which in turn gives access to valuable knowledge and networks. This institutional advantage means that international postdoctoral mobility has a positive influence on academic performance. The lack of significance of the correlation between precocity and international postdoctoral mobility makes it impossible to determine whether international postdoctoral mobility is a non-early advantage with positive effects on a scientist's productivity and career development.