A wave-guide capillary cell is applied as a spectrophotometric detector in the flow-injection determination of copper(II). The copper solution is injected and sandwiched between zones of diethyldithiocarbamate (DDTC) solution in the carrier flow line. The Cu-DDTC complex formed is then extracted into carbon disulfide by means of an extraction coil followed by a teflon-membrane phase separator. Because carbon disulfide has a refractive index which is sufficiently higher than that of the fused silica from which the capillary cell is made, the light from the source propagates inside the capillary via total reflection at the inner cell wall as in an optical fiber. The detection limit for copper in the proposed system is 20 ng ml −1 with linear response up to 1 μg ml −1.