Solar occultation observations of the Martian atmosphere near the limb of the planet were performed during the Phobos mission by means of the Auguste infrared spectrometer in the ranges 2707-2740 and 5392-5272 cm-1 with a resolving power of ≈ 1300. The spectra exhibit features at 2710 and 2730 cm-1 which have not been identified previously. After applying a set of corrections to the data and examining the spectra of various molecules, we are led to conclude that the best candidate for the above-mentioned features is formaldehyde (CH2O). It was observed in eight of the nine successful occultation sequences, mainly between 8 and 20 km with an average mixing ratio of 0.5+0.8-0.3 ppm (there are no good data below 8 km). The observations are performed in equatorial spring conditions. The altitude distribution of formaldehyde reveals correlation with the permanent haze opacity.