The average local ionization energy (I) over bar (r) and the Fukui function f(r) are conceptually-independent measures of molecular reactive behavior. In this Letter, we use Janak's theorem to establish a plausible link between (I) over bar (r) and f(r). In particular, both can generally be expected to identify the same sites as most reactive toward electrophiles. © 2005 Elsevier B.V. All rights reserved.