This paper deals with the effects on world welfare of sequential custo
ms union formation. A computational model of customs union formation i
s developed and simulated under a variety of assumptions. These assump
tions concern Various characteristics of the world, the pattern of cus
toms union formation, and GATT restrictions on common external tariffs
. The results show that unrestricted customs union formation is likely
to result in successive deterioration of world welfare, despite gains
for the member countries collectively. An examination of the current
GATT guidelines, however, suggests that if they were vigorously enforc
ed, world welfare would rise with customs union formation.