  Imprints of spinor-vector dualities have been uncovered in various string
constructions. They are typically induced by changing certain free general GSO
phases in the underlying string partition functions. This paper shows that
spinor-vector dualities in six dimensions are constrained by a fundamental
effective field theory consistency condition, namely that any six dimensional
low energy theory must be free of irreducible SO(2N) anomalies. Aspects of
spinor-vector dualities are analysed in four six-dimensional free fermionic
models which are distinguished by two generalised GSO phases. In addition, the
constraint on the number of spinors and vectors is confirmed on generic spectra
which may occur in K3 line bundle compactifications of the heterotic E8xE8
string.
