  We show that for a class of operator algebras satisfying a natural condition
the $C^*$-envelope of the universal free product of operator algebras $A_i$ is
given by the free product of the $C^*$-envelopes of the $A_i$. We apply this
theorem to, in special cases, the $C^*$-envelope of the semicrossed products
for multivariable dynamics in terms of the single variable semicrossed products
of Peters.
