The isomer formation of three isomers of C78 fullerene is discussed on
the basis of experimental evidence revealed by pressure-controlled fu
llerene production. Theoretically predicted most stable isomer, C2v'-C
78 has been found to be very sensitive to the graphite burning conditi
on. The fraction of C2v' isomer dramatically changes from zero to 50%
by changing the foreign gas pressure. The present findings strongly su
ggest that an isomerization within the C78 carbon network such as a py
racylene rearrangement type transformation unlikely takes place.