Parent cyclopropanes, annulenes, and related derivatives of C-60 and C-70 have been prepared and subjected to extensive spectroscopic scrutiny. Photolysis of the diazomethane adduct of C-60 afforded the C61H2 cyclopropane 2, accompanied by the isomeric annulene 4. Reaction of dimethyldiazomethane with C-60 led to C63H6 annulene 11, which upon heating was smoothly converted to the cyclopropane isomer 10 with an activation energy of 25 +/- 1 kcal . mol(-1). Addition of diazomethane to a toluene solution of C-70 generated a 12:1:2 mixture of pyrazolines. Photolysis of the mixture gave the C71H2 cyclopropanes 6 and 7, whereas thermolysis furnished the isomeric annulenes 8 and 9. Criteria have been developed for distinguishing fullerene cyclopropane derivatives from the corresponding annulenes via NMR and UV spectroscopy.