1. On the basis of consideration of the chemical and physical properties of unsaturated oxazolones, it is shown that the formation of true or pseudooxazolones depends upon the substituents of the oxazolone ring in positions 2 and 4, a structure with the most highly conjugated system being formed regardless of the method of formation. 2. 2-Benzylidene-4-halodimethyloxazolones, obtained by dehydration ofα,β-dihalo-α-phenacetylamino-propionic acids, are pseudooxazolones. 3. It is shown that 2-benzylidene-4-halomethyl-pseudooxazolones react with nucleophilic agents in different ways depending upon the degree of their nucleophilicity. Strong nucleophiles add on to 2-benzylidene4-halomethyl-pseudooxazolones in the 4,2'-position (1-4-addition), and the ring of the resultant saturated oxazolone at once opens with formation ofα-substituted derivatives ofβ-halo-α-phenacetylamiaopropionic acids. Weak nucleophiles, incapable of nucleophilic attack at the carbon atom in position 4 of the oxazolone ring, bring about cleavage of the ring at position 2. This process leads ultimately to formation of pyrrole derivatives.