The equilibria for the synthesis of C2H5OC(CH3)(3) (A) and C2H5OC(CH3)2(C2H5) (B) from C2H5OH (C) and CH2:C(CH3)(2) (D) or CH3CH:C(CH3)(2) (E) in the liquid phase were investigated at temperatures from 313K to 412K. On the basis of experimental equilibrium constants found for n(C)/n(D) or n(C)/n(E)greater than or equal to 4, the following values were obtained for Delta(r)H(m)(o)/(kJ . mol(-1)) and Delta(r)S(m)(o)/(J . K-1. mol(-1)) : (C+D=A), -(35.45+/-1.94) and -(82.37 +/- 5.99);(C+A=B), -(34.13 +/- 0.81) and -(87.82 +/- 2.18). The following values of Delta(r)H(m)(o)(298.15K)/(kJ . mol(-1)) have been derived : C2H5OC(CH3)(3)(1), -(350.8 +/- 2.6); C2H5OC(CH3)(2)(C2H5)(1), - (379.8 +/- 1.4).