AbstractBy treating 4‐chloro‐2‐phenylpyrimidine‐4‐14C with potassium amide in liquid ammonia, labelled 4‐methyl‐2‐phenyl‐s‐triazine was obtained. By degradation of thiss‐triazine derivative by acid hydrolysis, the14C‐label was found to be present in one position only,viz. position 4. This result strongly indicates that in this ring‐transformation no bicyclic intermediates are involved and that the opening of the pyrimidine ring occurs by fission of the C(5)C(6)bond and not of the C(4)C(5)bond. The mechanism