The impact of a commercially available cold flow improver additive (IA) on crystallisation within a model fuel system of eicosane in toluene solutions is investigated. Polythermal crystallisation method utilising turbidometric detection is used to assess the influence of IA addition upon eicosane solubility and nucleation inhibition at different solution IA treat rates, ranging from 0.1 to 10 wt%. Whilst IA is found to have negligible effect upon eicosane solubility, its nucleation is found to be disrupted, resulting in a reduction in crystallisation temperatures as a function of higher IA treat rate, with IA and eicosane phase separation also taking place at treat rates of 2 wt% IA and above. A change from an instantaneous to a progressive nucleation mechanism of eicosane in the absence and presence of IA within solution, respectively, is found. This behaviour is rationalised through the proposition that IA is incorporated within developing eicosane prenucleation clusters, hence disrupting their ability to readily grow to a critical size for nucleation. Nucleation kinetic analysis confirms that at given solution undercoolings, higher IA treat rate reduces the rate of nucleation due to an increase in the effective interfacial tension to nucleation, in line with a higher degree of thermodynamic control of the nucleation process, as might be expected for the progressive nucleation behaviour.