We study the mixing of neutral particles in quantum field theory: the neutral boson field and Majorana field are treated in the case of mixing among two generations. We derive the orthogonality of flavor and mass representations and show how to consistently calculate oscillation formulas, which agree with previous results for charged fields and exhibit corrections with respect to the usual quantum mechanical expressions.The study of the mixing of fields of different masses in the context of quantum field theory ͑QFT͒ has recently produced very interesting and in some sense unexpected results ͓1-14͔. The story began in 1995 when, in Ref. ͓1͔, the unitary inequivalence of the Hilbert spaces was proved for ͑fer-mion͒ fields with definite flavor on one side and those ͑free fields͒ with definite mass on the other. The proof was then generalized to any number of fermion generations ͓7͔ and to bosonic fields ͓2,5͔. This result strikes of the common sense of quantum mechanics ͑QM͒, where one has only one Hilbert space at hand: the inconsistencies that arise there have generated much controversy and it has even been claimed that it is impossible to construct a Hilbert space for flavor states ͓15͔ ͑see, however, Ref. ͓6͔ for a criticism of that argument͒.In fact, not only the flavor Hilbert space can be consistently defined ͓1͔, but it also provides a tool for the calculation of flavor oscillation formulas in QFT ͓3,8 -14,16 -18͔, exhibiting corrections with respect to the usual QM ones ͓19,20͔.From a general point of view, the above results show that mixing is an ''example of nonperturbative physics which can be exactly solved,'' as stated in Ref. ͓13͔. Indeed, the flavor Hilbert space is a space for particles which are not on shell and this situation is analogous to what one encounters when quantizing fields at finite temperature ͓21͔ or in a curved background ͓22͔.In the derivation of the oscillation formulas by use of flavor Hilbert space, a central role is played by the flavor charges ͓9͔ and indeed it was found that these operators satisfy very specific physical requirements ͓6,8͔. However, these charges vanish identically in the case of neutral fields and this is the main reason why the study of field mixing has been limited up to now to only the case of charged ͑complex͒ fields. We fill here this gap by providing a consistent treatment of both neutral bosons and Majorana fermions. To keep the discussion transparent, we limit ourselves to the case of two generations.Apart from an explicit quantization of the neutral mixed fields, the main result of this paper is the study of the momentum operator ͑and of the energy-momentum tensor͒ for those fields. We show how to define it in a consistent way and by its use we then derive the oscillation formulas, which match those for charged fields. We also comment on its relevance for the study of charged mixed fields, where, in the case when CP violation is present, the charges present a problematic interpretation which is still not completely clarified ͓10,11,14͔.The paper is ...