The thermal decomposition of tert-butyl peroxyacetate (TBPA) and of tert-t-butyl peroxy-pivalate (TBPP) in n-heptane solution has been measured up to 2500 bar at temperatures between 120 and 190 degrees C for TBPA and from 110 to 145 degrees C for TBPP. Three experimental procedures, two discontinuous and one continuous, have been applied. The data from these independent techniques are in excellent agreement. The first-order decomposition rate coefficient, k(obs) at identical p and T is significantly higher for TBPP than for TBPA. The activation energies at 2000 bar are 125.4 +/- 3.9 kJ mol(-1) and 150.4 +/- 1.7 kJ mol(-1) for TBPP and TBPA, respectively. Significant differences are also seen in activation volume: Delta V-obs(not equal) of TBPP decomposition is rather small (3.0 +/- 0.9 cm(3) mol(-1)) and independent of pressure whereas the activation volume of TBPA is relatively large and clearly depends on pressure. Delta V-obs(not equal), (TBPA) at 1000 bar is close to 18 cm(3) mol(-1). The origin of these differences is discussed. The experimental material supports the view that TBPA and TBPP are the archetypes of thermal peroxyester decomposition via single-bond and via concerted two-bond scission, respectively. Arrhenius-type expressions are presented for the pressure range 100 to 2500 bar. They allow to estimate k(obs) of TBPA between 120 and 190 degrees C and k(obs) of TBPP between 70 and 145 degrees C.