Abstract: A mixing cuvette for mixing different substances introduced therein by high speed rotary mixing, the cuvette being of a relatively small capacity but defining a great filling height. The cuvette body is elongated and encloses an internal cavity defined in its upper portion by a peripheral side wall that merges into a polygonal multi-surface lower portion. This cavity is open at the top and closed at its bottom. The lower portion includes a measuring region with a pair of opposite planar parallel surfaces through which may be directed a photometric measuring beam.