The phase behavior change when fluids are in a confined system has been an important issue after shale reservoirs became the focus of new oil and gas resources. Most studies on phase behavior of confined fluid systems have focused on modeling pore size dependence upon critical properties with no direct experimental evidence. Direct bubble point measurements of hydrocarbon mixtures in two synthesized mesoporous materials are: provided in this work. Two different synthesized mesoporous silica materials, SBA-15 and SBA-16, having similar pore sites (namely, 4 nm), were used. Well-ordered nanopores with narrow pore size distribution characterized these synthesized mesoporous silica materials. Decane methane and octane methane mixtures in 90:10 molar ratios were employed. The phase diagrams of the hydrocarbon mixtures were generated using a commercial thermodynamic simulator. The bubble point pressure of bulk (no porous medium) mixtures of decane methane and octane methane and the bubble point pressures with mesoporous materials (SBA-15 and SBA-16) were measured experimentally. Experiments were also performed with micrometer-sized sand particles. The bubble point pressure results of the hydrocarbon mixtures in the mesoporous materials were lower than those in the bulk, while the bubble points with sand were closer to those with bulk measurements. The bubble point pressure with SBA-15 having the higher total porosity and inner porosity was slightly lower than that with SBA-16. The differences may also be attributed to the different pore morphologies in the two mesoporous materials.