Number densities of N and O atoms have been determined using NO titration in Ar-N2, Ar-N2-H2, Ar-O2 and Ar-O2-H2 flowing microwave (2 450 MHz) post-discharges at 300 and 1500 Pa. The NO titration scheme is discussed from a kinetics point of view and applied to the high dilution of molecular gases in argon. The N/N2 density ratio is enhanced by a factor 3 when small quantities of H2 are introduced in Ar-N2 discharges. The high O/O2 density ratio obtained in Ar-O2 post-discharges (0.5 to 0.6) are probably due to adsorbed H2O that inhibits surface recombination of O-atom. The effect of H2 addition in Ar-O2 microwave discharge at 1500 Pa is to decrease the O atom density by homogeneous reaction involving H atoms and OH radicals.