The second-order polarizability tensor (beta) components of polymethineimine oligomers have been computed with the semiempirical intermediate neglect of differential overlap (INDO) coupled either to sum-over-states (SOS) or finite-field real space approach. Our goal is to rationalize the sign reversal of the longitudinal beta component with chain length. The INDO results are shown to compare well with the values obtained from previous ab initio calculations. Using a four-state SOS model that reproduces correctly the evolution of beta with chain length, we demonstrate that the shape of the beta curve can be explained as resulting from the competition between two contributions of opposite signs. In the framework of a real-space approach, those contributions are found to correspond to bond charge and bond polarization phenomena.