It is proved that both amplitude and phase in instantaneous optical processes can be detected from the number density of the excited state populations of coherent carriers when excited with femtosecond laser pulse pairs. Experimental technique of phase spectroscopy is used for investigations of semiconductor GaAs and rare earth solids. Phase spectra of GaAs, crystal Nd:YVO4 and powdered Nd:YAG are obtained. Dephasing time of GaAs exciton is 190fs at 77K and the Nd:YVO4 and Nd:YAG are 300fs at room temperature.