Rhodium-catalyzed hydrogenation of alkynes was studied under conditions of low concentration of substrate and either one or two equivalents of hydrogen using pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS). Four sample vials of the same volume but of different diameter enabled the investigation of the effects of surface area on reaction rates. Running the same experiments with stirring on or off allowed further details to emerge, notably the obvious effects of convection thanks to the localized sampling technique employed. Modelling of the system allowed for a better understanding of the relative importance of the various factors in play (reaction rate, surface area, mass transfer, convection). These factors should be carefully considered when studying or optimizing reactions involving narrow reaction flasks, especially when those flasks are unstirred.