AbstractDecomposition of phenyl acridinium‐9‐carboxylate is monitored using electrogenerated chemiluminescence in a flow system. The formation of the pseudobase from the acridinium ester [AE] is described by rate =k′1[AE] +k″1[AE][OH−]0.5, wherek′1= 0.020 ± 0.006 s−1andk″1= 2.1 ± 0.8 (L/mol)−0.5s−1. Irreversible decomposition of the pseudobase is described by rate =k′2[AE][OH−], wherek′2= 20.1 ± 3.8 (L/mol s). These kinetic equations, plus measurement of variation in emission intensity for constant acridinium ester concentration, are used to predict the resulting emission intensity v. pH behaviour given various contact times (in the 0.25 to 25 s range) for the acridinium ester to be in an alkaline solution prior to initiation of th