A method and apparatus are provided for determining rates and mechanisms of reactions in solution. The method comprises mixing reactants together and then passing the reactants, after mixing, to an electrospray or other ion source. The apparatus is configured so that the reaction time can be determined. This can either be by way of a capillary of known length and volume extending from a reaction tee or other mixing device, so that the reaction time can be determined from the capillary volume and flow rate, and/or by way of a container of fixed volume from which the reactants pass. From the ion source, ions pass into a mass spectrometer, where a mass spectrum is measured. By varying the reaction time, and measuring the different mass spectra, the rate of reaction can be determined.