The subject of this theoretical study was the low excited singlet state S1 of formaldehyde and acetaldehyde and their radical dissociation. Investigation into the dissociation from the S1 state was carried out using ab initio method. The activation barriers were determined for formaldehyde and acetaldehyde with correction made for energy of zeroth vibrations to be 66.9 and 56.5 kcal/mole , respectively. The analysis of mechnisms of dissociation from S1 and S2 excited singlet states was done based on calculated constans of photodissociation and intercombination conversion rates as well as the pecular features of the potential surfaces of these aldehydes. A possibility of using the spectroscopy of photofragments for identification of the aldehydes in their mixtures was studied. Based on calculated constants of the dissociation rate Kd and Frank-Condon factors the signal of fluorescence CHO(2I) --> CHO(2A') + hn was estimated as a function of the photolytic wavelength.