Ultrafast time-resolved X-ray diffraction has been used to study the dynamics of coherent acoustic phonons in fs laser-excited Ge and Au, with the particular goal to clarify the interplay of the electronic and thermal pressure contributions. For semiconductors it is usually assumed that the electronic pressure is the dominant driving force. Our measurements reveal that in Ge the relative strength of the electronic pressure decreases with increasing laser fluence. Only for low fluences the electronic pressure dominates, while at high fluences the thermal pressure exceeds the electronic pressure. For the case of Au the data are well described within the established theoretical framework using the known values for those material parameters which determine the laser-induced pressure, namely the energy relaxation time and the electronic and lattice Grüneisen parameters.