This thesis examines two semiconductor photocatalysts for their potential as catalysts for environmental remediation and new probes molecules that may be used to elucidate the mechanisms behind how the catalysts function. Iodine modified TiO₂ catalysts were synthesized and annealed at various temperatures under both oxidizing and non-oxidizing atmospheres. Though annealing temperatures altered the oxidation state of the iodine present in the catalysts, annealing environment had little effect. The catalyst that had been annealed at 600 °C under nitrogen contained no iodine after annealing but looked and behaved similarly to its iodine-containing counterpart, which was annealed at 600 °C in air, which supports claims that oxygen vacancies and crystal defects are responsible for visible light absorption and catalytic activity, not dopants. BiOX (X= Cl, Br, I) catalysts were prepared using the sol-gel method. Though they all exhibited significant activity when degrading phenol, they lacked the ability to efficiently degrade other probes, suggesting that these catalysts are effective at degrading only a select group of probe molecules. When examining new probe molecules,the selectivity of hydroxylation of the distal rings of 4-phenylbenzoic acid, 4-phenylsalicylic acid, and 5-phenylsalicylic acid were determined using partial TiO₂-mediated photocatalytic degradation and photo-Fenton conditions. The hydroxylation regiochemistry behaves as qualitatively expected for an electrophilic reaction, given the assumption that 4-carboxyphenyl is a slightly electron-withdrawing substituent. Selectivity for hydroxylation of the distal phenyl in 4- and 5-phenylsalicylic acid is reversed, due to the reversal of the electronic demand, while adsorption to the TiO2 surface is assumed to be analogous for the two structures.