We refine the Whitehead torsion of a chain equivalence of
finite chain complexes in an additive category A from an
element of ~Kiso1(A) to an element
of the absolute group K1iso(A).
We apply this invariant to symmetric Poincaré complexes
and identify it in terms of more traditional invariants. In the companion paper (joint
with Ian Hambleton and Andrew Ranicki) this new invariant is applied to
obtain the multiplicativity of the signature of fibre bundles mod 4.