The quenching rate constants of argon atoms in the 4p′[1/2]0−(2p 1), 4p[1/2]0−(2p 5), 4p[3/2]2−(2p 6) and 4p[5/2]2 − (2p 8) states, the upper levels of the 750.4, 751.5, 763.5 and 801.5 nm lines respectively, by H2, CH4, C2H2, C2H4, C2H6, CF4, CHF3 and SF6 have been determined. Argon atoms at room temperature are two-photon excited in the 184–190 nm range, and the decay time of the fluorescence emission is measured in the presence of various reagents. With H2, the collisional quenching rate coefficients of Ar(2p 1), Ar(2p 5), Ar(2p 6) and Ar(2p 8) are 0.27, 4.1, 6.8 and 5.4 × 10−10 cm3s−1, respectively. Therefore, in hydrogen rich plasmas, the 750.4 nm line of argon is the best suited for actinometry measurements. For the other studied molecules, the quenching rates range between 1.6 and 11 × 10−10 cm3s−1. For a correct estimation of the relative density of transient species in plasma by the actinometry technique, it is recommended to consider the collisional quenching of the upper states of the optical emission lines used.