A method for determining mixing times in a transparent stirred vessel for highly viscous fluids has been developed. The method involves a chemical decolourization with two acid-base indicators and a colourimetric diagnosis of digital images, deduced from a video captured during the mixing process. The advantages of the method developed are:(1) there is no disturbance of the fluid flow, and (2) measuring and determination of the degree of homogeneity is relatively effortless and cheap since it requires only a standard colour camera and commercial software for digital image analysis. This paper describes protocols for carrying this method out. In addition, examples of experimental results obtained for a standard impeller (of the helical ribbon agitator type) and a new piece of mixing equipment are given and discussed.