Diluted bitumen (dilbit), an oil sands product, may present new response challenges differing from conventional crude oil in terms of its potential environmental impacts. Simple naphthenic acids (NAs), a complex group of monocarboxylic acids, with a general formula C_nH_2n+zO₂, may be present in the source bitumen or may be created by photolytic weathering. Knowing the composition and concentrations of NAs created during the photo-degradation process of dilbit will help understand the fate, behavior and toxicity of dilbit. In the present study, two diluted bitumen products, Cold Lake Blend (CLB) and Access Western Blend (AWB), were mixed with saltwater and irradiated with natural solar light (Ottawa, Canada, 45.4°N) over winter and summer seasons, to assess the impact of sunlight on the chemical fate of the dilbit. For comparison, a light, sweet crude oil was exposed under similar conditions. The samples were analyzed by high performance liquid chromatography-high resolution mass spectrometry to examine the molecular transformation of diluted bitumen by solar irradiation. The abundances of NAs in all three test oils increased significantly after 90 days of solar irradiation, strongly suggesting that polar NAs were formed by photolysis. Further, greater increases in NAs in the light crude were found than in the two dilbits. Similarly, the lighter oil had higher photolytic removal rates of petroleum hydrocarbons than the two dilbits. The concentrations of NAs in oils exposed during the summer were generally higher than those exposed in winter. During summer exposure, the abundance of total NAs increased up to the 30-day’s solar exposure, then fell again, indicating the transient nature of these compounds. However, net increases in polar NA compounds were observed for all the winter exposed samples. Greater increases were observed in the smaller NA compounds (average C-number decreased), also accompanied by an increase in saturation (average z-number decreased). These chemical changes strongly indicate the effect of sunlight on the potential behaviour, fate and effects of spilled oil, with creation of new resin group compounds and reduction of aromatics and saturates. These changes may affect the viscosity of the oil and its ability to uptake water. These chemical compositions also imply significant changes to the ecological effects of the oil following a spill when aged in sunlight.