Abstract Although most of the parameters affecting spiral jet milling have already been studied, there are no investigations of several parameters and their effects on the flow conditions over the entire cross-section inside the spiral jet mill via non-invasive methods. In order to vary several geometrical and operative parameters and to determine the flow conditions inside the jet mill, a new type of experimental spiral jet mill apparatus was designed and constructed. It has an almost entire optical accessibility and its highly modular construction enables a very convenient variation of the geometric parameters. This paper describes the new type of mill apparatus and preliminary experimental results concerning the flow conditions inside the spiral jet mill as well as grinding effects under different conditions on particle size distributions of barium sulphate micro particles.