The Orion Perchlorate Ion Activity Electrode also is sensitive to perrhenate and thiocyanate activities. Response is Nernstian over several orders of magnitude, with detection limits about 10−6Ffor perrhenate and 10−5Ffor thiocyanate. Titrations can be carried out with ±0.5% relative standard deviations at the 10−1to 10−2Fconcentration level. Titration curves for addition of mercury (II) to thiocyanate, chloride, bromide, and iodide are U-shaped. The electrode appears to be highly sensitive to the anionic mercury-halide complexes formed in the early stages of these titrations. The data are consistent with the stabilities of the complexes.