This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon an atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the H alpha(656.3 nm) and H beta(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratio P-H2/P-Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar+ and H-2 which lead to ArH+ and H-2(+) ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the election temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H+, H-2(+), and H-3(+)) or excited species H(n = 2) with H-2 producing H atoms.