Motivated by the recent interest in using nitrites for organic modification of semiconductor surfaces, we have used density functional theory and cluster models to study the reactions of nitrites on semiconductor surfaces. In particular, we have calculated the energetics for the surface reactions of acetonitrile, 2-propenenitrile, and 3-butenenitrile on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces. For acetonitrile, our calculations predict that although it forms multiple products on the Si(100)-2 x I surface, the molecule does not react on the Ge(100)-2 x 1 surface. For 2-propenenitrile, the calculations show that whereas the C=C [2 + 2] cycloaddition product is thermodynamically most stable, formation of the hetero-[4 + 2] product is kinetically favored. We also have investigated the possibility of surface-catalyzed isomerization of 3-butenenitrile to 2-butenenitrile on the Si(100)-2 x 1 and Ge(100)-2 x 1 surface. We find that there are multiple surface-catalyzed isomerization pathways on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces. However, our calculated kinetics predicts that the amount of 2-butenenitrile formed by surface-catalyzed isomerization is small.