Apparatuses and methods are disclosed for the analysing of non-Newtonian fluids and determining parameters to characterise the relationship between shear stresses and shear rates, i.e. the parameters of one or more rheological models of viscosity. A plurality of viscosity measurements are made in a fluid using one or more vibratory transducers operating at a plurality of frequencies. Parameters for the fluid are determined based on the measured viscosity values and the frequencies of vibration at which the viscosity measurements were obtained.