A theory is proposed to study time-resolved x-ray diffraction on the pico- and subpicosecond time scales. Electromagnetic fields are treated in the frame of Maxwellian electrodynamics, whereas the molecular system is treated by using quantum mechanics. An expression is given for the time-resolved x-ray signal; it involves a three-time correlation function of the Fourier transformed electronic density and of the electric dipole moment of the system. This theory is applied to the study of the recombination of photodissociated iodine molecules in solution. Both geminate and nongeminate recombination are considered. The feasibility of the real time visualization of atomic motions is discussed.