The infrared spectra of the 32S, 34S, 16O, 18O isotopic species of SO2 trapped in solid krypton matrices at 20°K are reported. Under conditions of medium resolution (∼0.8 cm−1), single bands are observed in each of the spectral regions which correspond to the three vibrational modes. Analysis of all the matrix data on the basis of a valence force field which includes anharmonic corrections and in which the bond angle is treated as a variable parameter yields force constants very close to those derivable from the available gas-phase data. The SO2 bond angle which leads to the best fit of the experimental matrix data is 119°37′, in excellent agreement with the angle determined from microwave data. The use of measured isotopic shifts for the determination of the bond angles in triatomic molecules both from matrix and gas-phase studies is examined in considerable detail. It is shown that the use of observed frequencies in calculations involving two isotopic species can lead in many cases to reliable upper and lower limits of the bond angle. The quality of these limits depends on the magnitude of the anharmonic corrections and the stretch–bend interaction. The infrared spectra of matrix isolated SO2 and NO2 under conditions of higher resolution (0.3 cm−1) reveal some interesting features in the asymmetric stretching mode ν3. These features are discussed in terms of a hindered rotation in the solid krypton matrix.