Critical properties of both binary mixtures (hexane + methanol, hexane + carbon dioxide, and methanol + carbon dioxide) and a ternary mixture (hexane + carbon dioxide + methanol) were measured by using a high-pressure view cell with visual observation. The critical lines of the binary mixtures are continuous over the whole composition range between the critical points of the two pure components. The critical pressure of the binary mixtures of hexane + carbon dioxide and methanol + carbon dioxide shows a maximum, while the critical temperature of the binary mixture of hexane + methanol shows a minimum with the composition.