We have examined the microstructure, solution processing, thermal transitions, and; mechanical properties of regioregular poly(nonylbithiazole) (PNBT). PNBT Can adopt three distinct colors in the solid state-yellow, red, and metallic green-depending on processing conditions such as polymer concentration, solvent evaporation rate, temperature, and pressure. Microstructural variations were observed:between samples of different colors, including the intensification of optical absorption with the degree of crystalline order and/or crystal size. Yellow samples lacked well-developed three-dimensional order, whereas red and green samples were semicrystalline. The development of a lyotropic mesophase was observed at intermediate concentrations in a "diffusion couple" geometry. A crystal model with two planar, pi-stacked chains per unit cell was shown to be consistent with the experimental diffraction data. The simulated orthorhombic unit cell parameters are a = 2.38 nm, b = 0.72 nm, and c = 0.79 nm.