We show how to compute form factors, matrix elements of local fields,
in the restricted sine-Gordon model, at the reflectionless points, by
quantizing solitons. We introduce (quantum) separated variables in whi
ch the Hamiltonians are expressed in terms of (quantum) tau-functions.
We explicitly describe the soliton wave functions, and we explain how
the restriction is related to an unusual hermitian structure. We also
present a semi-classical analysis which enlightens the fact that the
restricted sine-Gordon model corresponds to an analytical continuation
of the sine-Gordon model, intermediate between sine-Gordon and KdV.