The decomposition after quench of the binary solid solution Al-6.8 at % Zn has been studied by transmission electron microscopy and neutron small angle scattering. The kinetics of formation of the Guinier-Preston zones has been followed at various aging temperatures inside the miscibility gap. Although the rate of decomposition increases rapidly with the degree of supersaturation no sharp spinodal transition is observed. The results are finally compared with the classical theory and with recent microscopic models