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 this s‐triazine derivative by acid hydrolysis, the 14C‐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 of this reaction is discussed.