In this work, the secondary structure of DNA in a water-salt solution and films of different humidity is studied using Fourier transform infrared spectroscopy. The spectra were recorded continuously during the drying of a drop of DNA solution on the crystal of the ATR attachment with a flow of dry nitrogen. The amount of water in the samples under study is such that the IR spectra at the initial stage of drying contain only spectral bands characteristic of vibrations of the water molecule. In the IR spectra of films with a low water content, lines of the IR spectrum of DNA are observed, which change as the humidity of the films decreases. These changes can be interpreted as signs of a B-A transition in the secondary structure of DNA. One of the main markers of the B-A transition is the position of the peak caused by asymmetric stretching vibrations of phosphate groups PO2-: the position 1225-1220 cm-1 corresponds to the B-form, and 1245-1235 cm-1 to the A-form.