High-pressure solution behavior such as density and phase behavior is a critical fundamental property for the design and optimization of various chemical processes, such as distillation and extraction in the production and purification of oils, polymers, and other natural materials. In this PhD study, solution behavior data are experimentally determined and equation of state (EoS) modeled for n-hexadecane, n-octadecane, n-eicosane, methylcyclohexane, ethylcyclohexane, cis-1,2-dimethylcyclohexane, cis-1,4-dimethylcyclohexane, trans-1,4-dimethylcyclohexane, o-xylene, m-xylene, p-xylene, and 2-methylnaphthalene at temperatures to 525 K and pressures to 275 MPa. A variable-volume view cell coupled with a linear variable differential transformer is used for the high-pressure determination. The reported density data are less than 0.4% of available literature data, which is within the estimated accumulated experimental uncertainty, 0.75%. Special attention is paid to the effect of architectural differences on the resultant high-pressure solution behavior. The reported data of low molecular weight hydrocarbons are modeled with Peng-Robinson (PR) equation of state (EoS), high-temperature high-pressure volume-translated cubic (HTHP VT-cubic) EoS, and perturbed-chain statistical fluid theory (PC-SAFT) EoS. The three pure-component parameters in PC-SAFT EoS can be either obtained from literature or from a group contribution (GC) method. Generally, PR EoS gives the worst predictions and HTHP VT-cubic EoS provides modest improvements over the PR EoS, but both of the equations underpredict the densities at high pressures. On the other hand, PC-SAFT EoS, with parameters from the literature or from a GC method, gives the improved density predictions with respect to PR EoS and HTHP VT-cubic EoS, although an overprediction of densities is found at high pressures. Model calculations also highlight the capability of these equations to account for the different densities observed for the hydrocarbon isomers. However, none of the EoS investigated in this study can fully account for the effect of isomeric structural differences on the high-pressure densities. For a better prediction of densities at high pressures, a new set of PC-SAFT pure-component parameters are obtained from a fit of the experimental density data obtained in this study and the mean absolution percent deviation is within 0.4%. The experimental technique and PC-SAFT EoS modeling method are extended to a star polymer-propane mixture. Star polymers with a fixed number of arms have a globular structure that does not promote chain entanglements. Star polymers can be synthesized with a large number of functional groups that can be readily modified to adjust their physical properties for specific applications in the areas of catalysis, coatings, lubrication, and drug delivery. In this study, a star polymer with a divinylbenzene core and statistically random methacrylate copolymer arms is synthesized with reversible addition-fragmentation-transfer method and fractionated with supercritical carbon dioxide and propane to obtain fractions with low molecular weight polydispersity. The phase behavior and density behavior are experimentally determined in supercritical propane for fractionated star polymers and the corresponding linear copolymer arms at temperatures to 423 K and pressures to 210 MPa. Experimental data are presented on the impact of the molecular weight, the backbone composition of the lauryl and methylmethacrylate repeat units in the copolymer arms, and the DVB core on the polymer-propane solution behavior. The star polymer is significantly more soluble due to its unique structure compared with the solubility of the linear copolymer arms in propane. The resultant phase behavior for the two homopolymers and the copolymers in propane are modeled using the PC-SAFT and copolymer PC-SAFT EoS, which give reasonable predictions for both phase behavior and density behavior. Model calculations are not presented for the phase behavior of the star polymers in propane since the PC-SAFT approach is not applicable for star polymer structures.