Similarly, amido-derivatives can be obtained from sulphur trioxide. The action of ammonia on that compound yields ammonium sulphamate, , or, if less ammonia be used, sulphamic acid, HO-(SO₂)-NH₂; they are both crystalline, soluble compounds.

The action of sulphur dioxide on ammonia is accompanied by the production of the analogous compounds, ammonium sulphurosamate and sulnhurosa/mie acid, the latter of which has the formula

These compounds may be taken as instances of bodies analogous to acids, in which the hydroxyl is replaced by the amido-group. They are, as a rule, stable in presence of water, and they do not generally unite with acids, the acidic nature of the oxygen which they contain counteracting the basic nature of the amido-group. Many compounds are however known, in which the amido-group replaces hydroxyl, and which, having no acidic oxygen present, are known only as salts in combination with acids. Some of these will now be described.