The calibration response of a diquat bis(tetra-4-chlorophenylborate) (DQT. 2T4C1PB) based ISE for DQT was stable for 55 days. The ISE calibration slope and minimum detectable activity a(min) of DQT were in the ranges (S.D. in parentheses) 30(0.6) to 28(1.7) mV/log a (DQT) and 4(3) 10-9M to 3(0.2) 10-6M, respectively. The times for 100% response were 3 s at 1 mM and 25 s at 1 μM and the ISE was useable at pH 2 to 12 and over a sample temperature range of 2 to 50C. Samples (2.54-244 μM DQT) could be determined using the standard addition method with about a -5% error and a precision of 7 to 8%. For comparison, DQT was also determined by titration with sodium tetraphenylborate (NaTPB) using DQT. 2TPB and TBA. TPB ISEs as TPB sensors. The errors were in the range of 12 to 20% for DQT analysis in deionized water, sodium chloride solution or simulated serum.