A device for the measurement of concentration fluctuations in flowing gases which can be used to establish and control the degree of mixing is described. The design is based on an improved laser‐schlieren technique combined with photometric detection. The device operates on‐line and has high sensitivity and fast response. A quantitative sensitivity evaluation is made and checked experimentally. The device is used to characterize the performance of some simple mixing configurations. The detection limits for various gas mixtures are given.