Let K be a field of characteristic not 2 with a non-trivial involution. It is well-known that every hermitian space V over K has a decomposition into an orthogonal sum of lines. Now suppose further that K is the quotient field of a Dedekind domain ?, and that L is an ?3-lattice on the hermitian K-space V. What can be said about the decomposability of L? If K is a local field and ? its ring of integers, then it is not difficult to show that every ?-lattice splits into 1and 2-dimensional components (cf. Jacobowitz [6] and Johnson [8] for details and applications). In this paper we consider the global theory of hermitian forms, and onr main objective is to prove the following analogue of the above local result: Let K be an algebraic number field with a non-trivial involution, and let D be the ring of integers of K. Suppose V is a regular indefinite herinitian space over K of dimension n ? 5. Then every Z-lattice L on V has a splitting L = L1 1 I Lt, with rank Li ?4 for l?i?t. The decomposition of quadratic forms was investigated in [13] and [4]. In [13] Watson showed that every indefinite quadratic form over Q of rank n ? 12 has a decomposition over Z. (Watson has also shown the number 12 to be best possible.) In [4] the present author extended Watson's result to quadratic forms over the flasse domains of global fields. In particular it was shown that given any algebraic number field K with ring of integers ?,7 there exists a natural number n (S) suich that every ?-lattice L with rank L ? n(s) splits non-trivially if its underlying quadratic space is indefinaite. But it was also seen that n (O) depends on the choice of ?) and ma;y be arbitrarily large. More explicitly, given any m > 0, there exist K and i such that there is an indecomposable ?-lattice on some indefinite quadratic Kspace of dimension greater than m. Thus our present result for hermitian forms is much stronger than that for quadratic forms. With regard to computation, herimitian forms are more manageable than quadratic forms, chiefly because dimension and discriminant suffice as local fractional invariants (the