Gaseous isomolecular equilibria of the type CuX + Ag=Cu + AgX, where X=F, Cl, Br, and I, were studied by effusion-beam mass spectrometry at elevated temperatures, and the differences between the dissociation energies of the CuX and AgX molecular species were determined with relatively high accuracy from thermochemical analysis of the equilibrium data. Analysis of literature data, plus new information on AgBr, yielded accurate values of D degrees(0) in kcal mol(-1) for CuF (102.0), AgCl (74.4), AgBr (66.4), and AgI (59.7), from which values were derived for AgF (81.5), CuCl (89.6), CuBr (79.2), and CuI (69.4), all +/- 1 kcal mol(-1). The result is a consistent set of dissociation energies for all eight of the Cu and Ag monohalides that will be useful in checking the reliability of quantum chemical calculations for these molecular species containing elements of increasing atomic number. Also, the isomolecular exchange equilibrium between CuF and NiF was studied in a similar fashion, leading to D degrees(0)( NiF)=104.4 +/-1.4 kcal mol(-1).