Remarkable tuning of reaction rates has been achieved for the isomerization of several dye molecules in supercritical fluid solvents using both small pressure changes and small additions of cosolvents. Rates of the thermal cis-trans relaxation were measured spectroscopically following irradiation for three dyes in supercritical carbon dioxide and ethane, pure and with several polar and protic cosolvents. The results are explained in terms of local density and composition enhancements, as well as by a change of mechanism due to strong hydrogen bonding. This study demonstrates the versatility of supercritical fluid solvents both to examine reaction mechanisms and as a means to tune rates.