2,3-Diphenylindenone oxide 1 undergoes thermal 1,3-dipolar cycloadditions via a carbonyl ylide with symmetrical olefinic dipolarophiles to give (i) with cis addends mixtures of endo and exo adducts in which endo adducts generally predominate in agreement with results obtained with 1-cyclohexyl-6-(cyclohexylimino)-1a-phenylindano [1,2-b]aziridine and (ii) with trans addends adducts in which the 2-exo-3-endo product predominates. The pyrilium oxide 4 is efficiently trapped with benzyne. 1,3-Diphen-ylisobenzofuran reacts with cis-addends to give endo–exo mixtures in which the endo adducts generally predominate. 2-Acetyl-5,5-pentamethylene-2-phenyl-Δ3-1,3,4-oxadiazoline undergoes regiospecific cycloaddition via a carbonyl ylide with unsymmetrical olefinic dipolarophiles in accordance with substituent electronic effects, and with 1,4-naphthoquinone a previously postulated type of intermediate adduct is isolated.