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Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for reaction monitoring. Several devices have been recently developed for online monitoring, using flow NMR, of batch reactions carried out under realistic experimental conditions in terms of, e.g., agitation and temperature. Here we show that time series of single-scan ultrafast 2D NMR (UF2DNMR) spectra can be collected to monitor solution mixtures that circulate in a flow unit. Fast multidimensional NMR methods have the demonstrated potential to provide kinetic and mechanistic information on reactions. UF2DNMR makes it possible to collect 2D data sets in less than one second, but relies on spatial encoding (SPEN) that is sensitive to sample motion, thus making online monitoring challenging. We characterize the interference between flow and spatial encoding and provide pulse-sequence- and hardware-based solutions. The resulting method is illustrated with the monitoring of a saponification reaction. UF2DNMR with a flow unit is a promising tool for the online monitoring of organic chemical reactions. [ABSTRACT FROM AUTHOR]