Diaquacobinamide (H2O)(2)Cbi(2+) or its conjugate base hydroxyaquacobinamide (OH(H2O)Cbi(+))) can bind up to two cyanide ions, making dicyanocobinamide. This transition is accompanied by a significant change in color, previously exploited for cyanide determination. The reagent OH(H2O)Cbi(+) is used in excess; when trace amounts of cyanide are added, CN(H2O)Cbi(+) should be formed. But the spectral absorption of CN(H2O)Cbi(+) is virtually the same as that of OH(H2O)Cbi(+). It has been inexplicable how trace amounts of cyanide are sensitively measured by this reaction. it is shown here that even with excess OH(H2O)Cbi(+), (CN)(2)Cbi is formed first due to kinetic reasons; this only slowly forms CN(H2O)Cbi(+). This understanding implies that CN(H2O)Cbi(+) will itself be a better reagent. We describe a single valve merging zone flow analyzer that allows both sample and reagent economy. With a 50 cm liquid core waveguide (LCW) flow cell and an inexpensive fiber optic - charge coupled device array spectrometer, a S/N = 3 limit of detection of 8 nM, a linear dynamic range to 6 mu M, and excellent precision (RSD 0.49% and 1.07% at 50 and 100 nM, respectively, n = 5 each) are formed. At 1% carryover, sample throughput is 40 h(-1). The setup is readily used to measure thiocyanate with different reagents. We demonstrate applicability to real samples by analyzing human saliva samples and hydrolyzed extracts of apple seeds, peach pits, and almonds.