We present an integrated liquid mixer/valve to be used for sample preparation for bioscience analysis systems. The mixer/valve is a glass-silicon bonded structure with a wafer-bonded cantilever-plate flapper valve and deep reactive-ion etched ports. It is passively pressure actuated and is distinguished by the fact that it can perform both a mixing and valving function simultaneously to mix two liquids noncontinuously. We present the design and fabrication of the mixer/valve and show that it successfully performs both its valving and mixing functions, including the discontinuous mixing of two liquids. We propose a method for characterizing mixing in this device using fluorescence microscopy and the pH dependence of fluorescein fluorescence. This method aims to allow one to extract the mixing length from a quantifiable observable. We present modeling and results of mixing length measurements using this method.