High molecular weight associating random copolymers, such as poly(fluoroacrylate-styrene) (polyFAST), are direct CO 2 thickeners that can dissolve readily in CO 2 and significantly increase viscosity. Although polyfluoroacrylate (PFA) is more CO 2 -soluble than polyFAST, PFA induces less thickening because it is nonassociating. High molecular weight poly(dimethylsiloxane) (PDMS) and poly(vinyl acetate) (PVAc) can increase CO 2 viscosity if toluene cosolvent is introduced. Bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) can thicken CO 2 modestly if ethanol cosolvent is added. Little CO 2 thickening has been reported for low molecular weight polymers, including poly(vinyl ethyl ether) (PVEE) and poly-1-decene (P1D). Associating oligomers of poly(propylene oxide) (PPO) with pendant aromatic groups showed modest CO 2 viscosity enhancement. Crosslinked phosphate esters can increase the viscosity of CO 2 if a very substantial concentration of a light alkane cosolvent is added. Indirect thickening of CO 2 involves the generation of high apparent viscosity, completely waterless CO 2 -in-oil (C/O) emulsions that can be attained when CO 2 is mixed with mineral oil containing a silicone-alkyl copolymeric surfactant; similar behavior was observed with a proprietary blend of oil and surfactants. Only one nanoparticle-based thickening of CO 2 involving the dispersal of dilute concentrations of nanoparticles that are surface-functionalized with highly CO 2 -philic groups was reported (however, no explanation was provided to explain how the high molecular weight polymer used in this study could be CO 2 -soluble). New experimental results were obtained for some readily available or easily synthesized CO 2 -thickening candidates. PolyFAST yielded very substantial viscosity increases. PFA, PDMS-toluene, and PVAC-toluene could appreciably thicken CO 2 . Very little detectable viscosity enhancement was achieved with low molecular weight PVEE or P1D. CO 2 was thickened slightly with AOT-ethanol. Regarding future studies, novel, nonfluorous associating CO 2 -philic oligomers, and highly CO 2 -philic phosphate esters that remain dissolved in CO 2 after cross-linking are the most obvious candidates for CO 2 direct thickeners.