Sydnone Ethylenes. III. Chemical Reactions of Excited 4‐Styryl‐sydnonesDependent on the character of the substituents in the photo reactions of 4‐styryl sydnonesZ/E‐isomerism competes with CO2‐elimination. Excited p‐nitrostyryl sydnones exhibit a relatively strong degradation of the bond order in the CC region. Their quantum yields ofZ/E‐isomerism are therefore much higher than those of compounds with p‐substituents of lower acceptor effect. In these compounds the cleavage of the sydnone ring with elimination of carbon dioxide is favoured especially if a methyl group is in 3‐posi