The infrared spectra of HNCO and HNCS are quite complicated and the band assignments are still disputed. In this note we try to add some arguments based on the normal coordinate analysis in favour of the recently proposed assignments1,2. The analysis was performed by the standard GF method with the frequency assignment due to Ashby and Werner1for HNCO, and Durig and Wertz for HNCS2. The frequencies only were used in the iteration procedure and the results are given in the Table.