It is now well established that the local density of a supercritical fluid (SCF) about a dilute solute may be significantly greater than the bulk density of the fluid. 1Solvatochromic probes have been particularly successful in demonstrating that this effect of enhanced solvent-solute interaction, often termed “local density augmentation” or “solvent clustering”, is most dramatic in the compressible and low-density subcritical regions of the fluid. 2 This phenomenon has also invokedthe possibility of solvent cage effects occurring in SCFs that are enhanced beyond those experienced in normal liquids. Recently, two cases of anomalous behavior in SCFs havebeen reported that havebeen attributed to such clustering effects. Randolph and Carlier reported3 enhanced bimolecular rate constants for Heisenberg spin-exchange between nitroxide free radicals in supercritical ethane. The results were explained in terms of increased collision times due to local density augmentation about the encounter pair. Bright and co-workers4 have attributed a decrease in the emissive rate of decay of pyrene excimer in SC C024a and C2H44b to a protective se-questering of the excimer by solvent clusters. In order to test thegenerality of such solventbehavior we have used laserflash photolysis (LFP) and time-resolved absorption to directly probe potential SC solvent clustering effects about diffusive encounter pairs. The TT annihilation (TTA) process of triplet benzophenone (3BP) and the free radical termination reaction of benzyl radical (PhCH2) have been chosen for study since each process is known to proceedthrough pure diffusion-controlled bimolecular reactivity that adheres to simple diffusion models in normal liquids. Additionally, insights into the integrity of SC solvent cages have been obtained by investigation of the mechanism of decomposition of the benzyl radical precursor, dibenzyl ketone, which complements the recently reported product studies5 of substituted dibenzyl ketones in SCFs.