Is enthalpy additive in mixing?
The enthalpy is not additive under isobar, adiabatic conditions for non-ideal mixtures.

The change in the enthalpy is called the excess enthalpy (enthalpy of mixing). It can be computed with models such as the Margules, van Laar, Wilson, NRTL, and UNIQUAC.

These model give equations for the excess Gibbs energy. The excess enthalpy can be derived from these as described in 6.2 Fundamental Relations of Excess Functions in Molecular Thermodynamics of Fluid-Phase Equilibria (3rd Edition). (F'x already referred to this excellent book.)

One has to be careful with the conflicting sign convetions in the literature.