Several formulations for component mode synthesis have been proposed in the literature, with the principal differences among them being the types of boundary conditions imposed on individual components at the interfaces where they later will be joined when component natural frequencies and modes are computed. Formulations based on component modes with fixed interfacial conditions, free interfacial conditions, and mixtures of fixed and free conditions have be en proposed. While all formulations yield reliable results when all component modes are included, modal truncation errors are introduced if incomplete sets of component modes are used. In this communication, a new procedure for determining modal truncation errors in modal synthesis formulations is presented and applied to two formulations, one with mixed interface conditions and one with fixed interface conditions. Errors in estimated natural frequencies and local errors in mode shapes are investigated for the case of a two component joined beam system. The results indicate that formulations using component modes with free interface conditions are inherently susceptible to modal truncation errors because of nonzero forces and moments acting at the joints after assembly.