A theory is developed of laser mode locking due to saturable absorbers when the dispersive property of the active material is taken into account. The electric field within the cavity is expanded in terms of cavity modes, the problem is treated under the rate-equation approximation, and only third-order nonlinear polarization terms of the dye are considered. The field is assumed to be made of2N + 1oscillating modes. It is shown that, owing to dispersion, the phase φlof the modes (lrunning from-Nto+N) has a term that is proportional to l2and a term (smaller than the previous one) that is proportional to l4. In agreement with the experimental results, the term proportional to l2increases the pulsewidth of the total electric field over the case of perfect mode-locking and it also gives a positive linear sweep of the field frequency. The pulsewidth increase is expressed by a factor γ that is proportional to the fourth power of the oscillating bandwidth, to the square of the relaxation time T1of the dye, and to the product of the length of the active material times a quantity ε that is related to dispersion of the material.