A microfluidic device with integrated waveguides and a long path length detection cell for UV/Vis absorbance detection is presented. The 750 mum U-cell detection geometry was evaluated in terms of its optical performance as well as its influence on efficiency for electrophoretic separations in the microdevice. Stray light was found to have a strong effect on both, the sensitivity of the detection and the available linear range. The long path length U-cell showed a 9 times higher sensitivity when compared to a conventional capillary electrophoresis (CE) system with a 75 mum inner diameter (ID) capillary, and a 22 times higher sensitivity than with a 50 mum ID capillary. The linear range was comparable to that achieved in a 75 mum ID capillary and more than twice as large as in a 50 mum ID capillary. The use of the 750 mum U-cell did not contribute significantly to band broadening; however, a clear quantification was made difficult by the convolution of several other band broadening sources.