Abstract: The invention relates to an apparatus and method for testing the properties of chemicals in a reaction flow stream. In the method, chemicals from the reaction flow stream are directed to the apparatus for testing the physical properties of the chemicals, which is an indication of the state of the chemical reaction. Data measured by the apparatus is stored in a computer, and the computer may be used to analyze the data and to control the reaction parameters based on the analysis of the data. The apparatus used for the analysis comprises a rheometer having an actuated shaft and a concentric container, wherein the shear properties of the material sampled in the apparatus are measured when the actuator shaft is oscillated within the container. Various unique configurations of the apparatus are described.