The intercepted solar radiation per plant (ISR) is one of the main environmental factors that affects oil concentration and quality in sunflower grains. A reduction of ISR during the 250-450ºC day after flowering period (the critical period) accounted for most of this effect on the oil percentage in the Dekalb G-100 traditional hybrid. However, a critical period for the effect of ISR on the percentage of oleic acid has not been defined. To fill this gap, two experiments were conducted at Balcarce, Argentina, during 2008-2011 using two hybrids with different grain filling period duration. The ISR was modified by both, shading (80%) and thinning plants (50%), either throughout grain filling or by applying short period treatments. The response of oleic acid concentration to accumulated ISR from R6 to physiological maturity (R6-PM) and for 100ºC day segments after flowering was analysed by fitting an exponential rise to maximum equation to experimental data. The critical period was defined by those 100ºC day segments where R² for the fitting was higher than the one obtained with ISR accumulated during R6-PM. High susceptibility period was found for the effect of ISR on the percentage of oleic acid. Critical periods for both hybrids were located between 350 and 600ºC day after flowering, i.e. during the second half of the grain filling period. The length of critical periods was different between both hybrids. These results could improve model prediction of oil quality of sunflower grown under different environmental conditions and generate a robust framework to assist plant breeding.