The liquid-phase additions of isopropyl alcohol to isobutene to give tert-butyl isopropyl ether (IPTBE) and of methanol to a mixture of 2-methyl-2-butene (2M2B) and 2-methyl-1-butene (2M1B) to give tert-amyl methyl ether (TAME) have been studied in a calorimetric reactor. The heat capacity of IPTBE and TAME and the enthalpy change associated with the IPTBE and TAME synthesis reactions have been determined. At 343 K, the standard molar reaction enthalpy for TAME formation from 2M1B is -34.9 +/- 2.5 kJ.mol(-1) and from 2M2B is -27.1 +/- 2.5 kJ.mol(-1). At 298 K, the standard molar enthalpy for IPTBE formation is -21.7 +/- 1.6 kJ.mol(-1). A determination of the apparent activation energy of 70.3-78.4 kJ.mol(-1) for the IPTBE synthesis has been performed graphically from the plots of heat flow rate vs time. TAME heat capacity in the liquid phase has been found to follow the equation C-p (J.mol(-1).K-1) = 1059.5-6.1271(T/K) + 1.1093 x 10(-2)(T/K)(2) and that of IPTBE the equation C-p (J.mol(-1).K-1) = 1181.5-6.7818(T/K) + 1.2186 x 10(-2)(T/K)(2). Comparison of thermodynamic and kinetic data obtained for the IPTBE system with that previously reported for tert-butyl methyl ether (MTBE) and tert-butyl ethyl ether (ETBE) is also performed.