We have developed a novel, transient response technique for liquid-phase heterogeneous catalytic studies, equipped with an electrospray ionization mass spectrometry (ESI-MS) detector. The technique was successfully applied as an online method for real-time detection of species dissolved in aqueous product streams at the exit of a catalytic reactor. Two test reactions, nitrite reduction with Pt/SiO2 and glucose oxidation with Pt/CNF/Ni, were used to demonstrate semi-quantitative monitoring of reactants, intermediates, and products. The capability of the novel technique is demonstrated by the fact that the ESI-MS detector is sufficiently sensitive to determine quantitatively extreme small amounts of physisorbing nitrite, down to 0.5% of a monolayer on the Pt surface. Nitrite also reacts with preadsorbed hydrogen, and the quantitative experimental results agree with the formation of both nitrogen and ammonia. The ESI-MS detector is able to distinguish between different components simultaneously as, in the case of glucose oxidation; this is its most significant advantage over existing transient techniques. A limitation of ESI-MS is its inability to detect gases dissolved in liquids, due to the relatively mild ionization process through electrospray. The second part of this article discusses an alternative detector to satisfy this deficiency. (c) 2008 Elsevier Inc. All rights reserved.