The general analysis of binary reactions involving particles with arbitrary spin is reformulated in such a way, that it applies equally well to relativistic particles (including photons). This is achieved by using longitudinal spin components ("helicity states") not only in the initial and final states, but also in the angular momentum states which are employed as usual to reduce the S-matrix to a simpler form. Expressions for the scattering and reaction-amplitude, intensity and polarization are given. They involve fewer vector-addition coefficients than the customary formulas, and no recoupling coefficients. The application to some examples is sketched, and in the Appendix some formulas are given that may be of use in the applications.