Cyanine-like dyes are promising candidates for third-order nonlinear optical (NLO) applications such as all-optical switching. Here, we examine, the consequences for linear and nonlinear optical properties of varying substituents on the central methine unit of bis(dioxaborine)-terminated anionic pentamethines and bis(indole)-terminated cationic heptamethines. The variation in absorption maxima and electrochemical potentials with structure can generally be rationalized using the Dewar-Knott rules, providing that mesomeric and inductive electron-withdrawal and donation are explicitly considered. In the case of nitro- and (dioxaborinyl)vinyl-substituted bis(dioxaborine) pentamethines, the low-energy transitions are significantly broadened, consistent with H-1 NMR spectra indicating deviation from cyanine-like geometry. Real and imaginary parts of the third-order polarizabilities, Re(gamma) and Im(gamma), were measured at 1.3 mu m. The values of Im(gamma) indicate that the values of Re(gamma) are significantly resonantly enhanced, while the positive value of Re(gamma) found for a nitro-substituted dioxaborine example is atypical for a symmetrical polymethine and suggests that a two-state treatment is inadequate. The relevance of these results to chromophore design for third-order NLO applications is discussed.