A fast micromachined mixer/reactor was employed in a continuous flow injection set-up for the direct detection of chromium(III) in aqueous samples by chemiluminescence (CL). With the device Cr(III) can be detected at a concentration of 10-7 M and a linear calibration curve was obtained from 10-6 to 10-4 M Cr(III). In the continuous flow injection mode the system can perform fully automated detection with a reagent consumption of only 43.2 mL per day. In addition, a general theory is presented for the performance of diffusion-based mixer/reactor devices. This is the first report of the use of a micromachined device for the quantitative detection of Cr(III) with CL. Simple instrumentation, easy operation and low reagent consumption make this system a potential candidate for the in situ monitoring of Cr(III).