The effect a transient heat flux has on in-tube boiling has not been studied extensively for some refrigerants commonly proposed for use in concentrated solar power organic Rankine cycle systems. In this study, the effect of abrupt step changes (upwards and downwards) in the inlet vapour quality to a flow-boiling test section on the heat transfer coefficient was considered. Tests were conducted with R-245fa at a saturation temperature of 35°C in an 800 mm horizontal smooth tube with an inner diameter of 8.31 mm and a constant test section heat flux of 7.5 kW/m2. Initial inlet vapour qualities ranged between base values of 0.15 and 0.40 with mass fluxes of 200 and 300 kg/m2s. Baseline heat transfer coefficients at steady-state conditions were determined, followed by a series of transient-state response investigations. For these, sharp upward and downward step perturbations of the inlet vapour quality were considered. It was found that for a step size magnitude of 0.13 in the vapour quality, the actual heat transfer coefficient differed from the expected quasi-steady-state heat transfer coefficients during the transient. During the downward step, it was 8.7 to 11.7% higher than the expected heat transfer coefficient, while during the upward step, it was 9.3 to 26.0% lower for a mass flux of 200 kg/m2s, depending on the initial inlet vapour quality. For a mass flux of 300 kg/m2s, it was 7.2% and 16.7% higher and 13.8 to 17.8% lower for the downward and upward step respectively.