Reaction kinetics studied by mass spectrometry (MS) has previously been limited to millisecond time resolution. This paper presents the development of a submillisecond time-resolved mass spectrometric method for fast reaction kinetic study, based on the capability of desorption electrospray ionization (DESI) for direct and fast ionization of a high-speed liquid jet stream. The principle underlying this methodology is that two reactant solutions undergo rapid mixing to produce a free liquid jet which is ionized by DESI at different positions corresponding to different reaction times. Due to the high velocity of the liquid jet, high time resolution can be achieved. In this study, the fast reduction reaction of 2, 6-dichlorophenolindophenol (DCIP) and L-ascorbic acid (L-AA) was chosen as an example to demonstrate this concept, and the reaction rate constant was successfully measured with an unprecedented time resolution of 300 μs. The good agreement of the measured value of (116 ± 3) s(-1) with that measured by the stopped-flow optical method (105 ± 2) s(-1) validates the feasibility of such a DESI-MS approach. Unlike classical spectroscopic techniques that require either chromophoric substrates or labeling, MS is a general detector with high chemical specificity. Therefore, this time-resolved DESI-MS method should find wide applications in fast (bio)chemical reaction investigations.