Gram Determinants of Type B_n Richard Dipper, Gordon James and Eugene Murphy Keywords Hecke algebras, Gram determinants. Status To appear. Abstract The Hecke algebra H = H_{R,Q,q}(W_n) over a commutative ring R with the two parameters Q,q associated with the Weyl group W_n of type B_n has certain distinguished representations afforded by modules S^{\lambda} labelled by bipartitions \lambda of n. These are precisely the irreducible H-modules, if H is semisimple. In general there is a symmetric H-invariant bilinear form defined on S^\lambda. We present here a formula for the determinant of the Gram matrix of S^\lambda.