The optical properties of high-concentration solutions of conjugated molecules are dominated by inner-filter and re-absorption effects. We apply a simple model to isolate the contributions from these phenomena to the non-linearity of the observed photoluminescence (PL) intensity with concentration. Poly(9,9-dioctylflourene) (PFO) solutions were studied across a range of concentrations and in a number of different solvents. The model accurately predicts the behaviour of PFO in good solvents up to concentrations above 0.1 mg/ml. In solutions of PFO in poor solvents and at high concentrations where we find a significant amount of β-phase PFO present the model is less accurate. We attribute the difference between predicted PL behaviour and that observed to non-radiative energy transfer from the bulk “glassy” phase PFO to the β-phase PFO.