A study of the reaction pathways of 1,1-diphenylethylene; methyleneindan; fluorenene; trans,trans-1,4-diphenyl-1,3-butadiene; cis,trans-1,4-diphenyl-1,3- butadiene and cis,cis-1,4-diphenyl-1,3-butadiene with singlet oxygen and the singlet oxygen mimic, 4-alkyl-1,2,4-triazoline-3,5-dione (RTAD) has been undertaken. Comparisons of the reactivity and reaction mechanisms among the isomers and electrophiles have been conducted. Singlet oxygen exhibits significantly lower reactivity compared to RTAD. RTAD reacts with 1,1- diphenylethylene to produce 4+2/ene and 4+2/4+2 (2:1) adducts but with methyleneindan to give a complex mixture including the ene adduct. Reactions of isomeric 1,4-diphenyl-1,3-butadienes with RTAD produce 4+2 adducts. The results of the studies provide fundamental mechanistic information which is important to a better understanding of the detrimental and beneficial reactions of 1O2 in biological systems and the environment.