The correlation between the photocatalytic degradability of polycyclic aromatic hydrocarbons (PAHs) over Pt/TiO₂-SiO₂in water and their quantitative molecular structure was studied. Six PAHs, namely, naphthalene, fluorene, phenanthrene, pyrene, benzo[a]pyrene, and dibenzo[a,h]anthracene, were tested in Pt/TiO₂-SiO₂suspension under UV irradiation. The results showed that the degradation efficiencies of the higher molecular weight PAHs were enhanced significantly in the presence of Pt/TiO₂-SiO₂, while the degradation efficiencies of the lower molecular weight PAHs were decreased in the presence of Pt/TiO₂-SiO₂. Both the photolysis and photocatalysis of all PAHs fit the pseudo-first-order equation very well, except FL. Quantitative analysis of molecular descriptors of energy of the highest occupied molecular orbital (Ehomo), energy of the lowest unoccupied molecular orbital (Elumo), and the difference betweenElumoandEhomo, GAP (GAP=Elumo-Ehomo), suggested that the GAP was significant for predicting a PAHs’ photocatalytic degradability. Through comparison against the maximum GAP (7.4529 eV) of PAHs (dibenzo[a,h]anthracene) that could be photocatalytically degraded and the minimum GAP (8.2086 eV) of PAHs (pyrene) that could not be photocatalytically degraded in this study, the photocatalytic degradability of 67 PAHs was predicted. The predictions were partly verified by experimental photocatalytic degradation of anthracene and Indeno[1.2.3.cd]pyrene.