In this work, a new in-line and non-invasive rheological instrument and measurement technique was evaluated and tested in an experimental flow-loop. The concept of the new rheometer is based on the Ultrasound Doppler Velocity Profile technique (UVP), in combination with a pressure difference method, (PD). Complex model fluids such as highly shear-thinning viscoelastic surfactant solutions, and non-Newtonian suspensions containing cellulose fibres or corn starch particles in a water or water-glucose matrix, were tested in the flow-loop. INTRODUCTION The rheological behaviour of the in general, non-Newtonian, highly concentrated and non-transparent fluids used in industry have so far been analysed using commercially available instruments, such as conventional rotational rheometers. Most of these commercially available instruments are usually not suitable for in-line rheological measurements and quality control as they are based on invasive methods, and have geometries that can affect and disturb the flow. Real flow situations can almost never be obtained using these instruments and they may also cause severe bio-safety problems due to contamination, which also limits their applicability in many situations. The research in the present study, continued the work made by Ouriev in a previous Ph.D project, using a modified experimental flow-loop at ETH – Institute of Food Science and Food Process Engineering in Zurich, Switzerland. The main objective of this research work was to expand and evaluate the UVP-PD methodology for non-invasive (no contact) rheological characterisation of nontransparent highly concentrated model fluids and shear thinning, viscoelastic, multiphase surfactant solutions. Aiming the adaptation of the UVP-PD method towards complex water-cellulose fibre suspensions introduced a new interesting field of application. UVP-PD IN-LINE MEASURING PRINCIPLES The principle of the Ultrasound Doppler Velocity Profile technique (UVP) is described extensively in previous work, e.g. by Takeda, and the combination with a pressure difference method, (PD) by Ouriev. Only a brief description of the method is given here. The rheological flow properties were derived from the simultaneous recording of the velocity profiles and the corresponding pressure drop within the pipe, using a new flow-adaptor with housing for an ultrasonic transducer and two pressure sensors. A schematic representation of the inline adaptor is given in figure 1.