A general framework for the solvatochromic behavior of pyridinium phenolate betaine dyes is presented, based on the variations with the medium of the electrophilic Fukui functions of their electron-pair donor and acceptor moieties. The model explains the 'anomalous' solvatochromic behavior of large betaines, which change their behavior from negative to inverted, when electron-pair donor and acceptor groups are separated by a conjugated chain of variable size. (C) 2012 Elsevier B.V. All rights reserved.