Electron localization in the methyl-ring-substituted derivative of polyaniline (PAN), poly(o-toluidine) (POT), has been investigated by transport and magnetic measurements. The HCl salt of POT shows greater electron localization than that of PAN due to decreased interchain diffusion rate caused by increased interchain separation and decreased interchain coherence. Each of these is attributed to the addition of one ${\mathrm{CH}}_{3}$ to each ${\mathrm{C}}_{5}$ ring. The general implications for control of dimensionality and conductivity are discussed.