Mixtures of acetaldehyde and water are reactive multicomponent systems because poly(oxymethylmethylene) glycols are formed. A study on the kinetics of the formation of these oligomers was carried out using a new microreactor NMR probe head that combines online flow H-1 NMR spectroscopy with microreaction technology. The study covers temperatures between 278 and 298 K and pH values between 3.5 and 10.3. From the peak areas in the H-1 NMR spectra, quantitative results for the conversion of acetaldehyde were obtained. On the basis of the new data, a reaction kinetic model was developed and numbers for the kinetic constants of poly(oxymethylmethylene) glycol formation were determined together with a correlation that describes their dependence on the temperature and pH value.