Using U.S. patent records in nanotechnology, we study the relationship between inventor mobility among firms and knowledge diffusion. We find evidence consistent with a story that, in one important nanotechnology subfield, when inventors move among firms they spread knowledge. In particular, we find that if we consider any two patents, A and B, where A and B are in the ``Chemicals, misc.'' subclass, A and B are assigned to different firms and where A is granted after B, patent A is more likely to cite patent B if the patent A firm employs an inventor who earlier worked for the patent B firm.