A new technique has been developed for assessing the chemical processes which take place during the formation of polymer coatings. It is capable not only of determining the reactions which take place but also when they happen. By programming the computer of a Fourier transform (or a computer controlled dispersive) infrared spectrometer to take and store spectra at frequent regular intervals, the progress of film formation can be followed in a manner similar to time-lapse photography. The computer can then be used to subtract one spectrum from another to show only the infrared band-shifts resulting from these reactions. By subsequently subtracting each spectrum from the previous one the sequence of reactions can be determined. The curing in air of a common oil-modified alkyd will be used to demonstrate this technique.