The invention relates to a method for the quantitative determination of the viscoelastic properties of a complex medium from the movement of particles in suspension, wherein a sample of the medium is placed in a device for measuring the light diffused by the particles in suspension, suitable for projecting coherent light onto said sample, and for detecting and analysing the diffused light in order to calculate the inter-image distance d2, said method including the steps essentially consisting of: a) characterising the opacity of the medium by measuring the transport length I* of said medium, b) inserting a sample of said medium into a flask with predetermined dimensions; c) placing the flask in a thermostatically controlled measurement chamber (1) of said measurement device; d) projecting a beam of coherent light onto the sample and detecting the light diffused by the particles in the form of a series of sequenced images; e) analysing the movement of the particles according to time by calculating the inter-image distance d2 and the ratio d2/d2max; f) establishing an analytic expression E by directly comparing, for the value of I* determined in step a), the ratio d2/d2max and the mean square displacement MSD, in order to calculate the viscous and elastic moduli of the medium. The invention also relates to a device for implementing said method.