The emission properties of polyindenofluorenes with various proportions of straight- and branched-chain alkyl substituents have been compared. The polymer with straight octyl substituents shows green emission due to formation of aggregates, while the polymer with branched 2-ethylhexyl substituents shows blue emission. Studies of the film morphology show the presence of ordered structures due to pi-stacking of the polymer chains for the octyl-substituted polymer, whereas the polymer with branched side-chains shows no such order. Copolymers show intermediate behavior. A clear correlation is established between the degree of straight-chain alkyl substitution, the formation of ordered structures in the films, and the amount of long wavelength emission in the solid-state spectra.