Pressure effects upon asymmetric photosensitization have been investigated for the first time in the enantiodifferentiating Z-E photoisomerization of cyclooctene (1), sensitized by chiral aromatic esters (2-7). The product＇s enantiomeric excess (ee) and E/Z ratio were critical functions of the applied pressure, exhibiting an unprecedented switching of the product chirality. Depending upon the chiral sensitizer employed, the differential activation volume (Delta Delta V double dagger) varies widely from -3.7 to +5.6 cm(3) mol(-1), which is unexpectedly large for an enantiodifferentiation in the excited state. However, the Delta Delta V double dagger values obtained do not correlate with the differential activation enthalpy (Delta Delta H double dagger) or entropy (Delta Delta S double dagger) obtained from temperature-dependence studies, indicating that pressure and temperature function as independent perturbants for the photoenantio-differentiation process. Further investigations on the pressure dependence of ee at low temperatures enable us to construct the first three-dimensional diagram that correlates the product＇s ee with pressure and temperature changes. The combined effects of temperature and pressure provide us with a versatile tool for the multidimensional control of asymmetric photochemical reactions, in which we can switch and/or enhance the product chirality at more readily accessible temperatures and pressures, without using antipodal sensitizers;