A rf microplasma jet working at atmospheric pressure has been characterized for Ar, He, and Ar/CH4 and Ar/C2H2 mixtures. The microdischarge has a coaxial configuration, with a gap between the inner and outer electrodes of 250 μm. The main flow runs through the gap of the coaxial structure, while the reactive gases are inserted through a capillary as inner electrode. The discharge is excited using a rf of 13.56 MHz, and rms voltages around 200–250 V and rms currents of 0.4–0.6 A are obtained. Electron densities around 8×1020 m−3 and gas temperatures lower than 400 K have been measured using optical emission spectroscopy for main flows of 3 slm and inner capillary flows of 160 SCCM. By adjusting the flows, the flow pattern prevents the mixing of the reactive species with the ambient air in the discharge region, so that no traces of air are found even when the microplasma is operated in an open atmosphere. This is shown in Ar/CH4 and Ar/C2H2 plasmas, where no CO and CN species are present and the optical emission spectroscopy spectra are mainly dominated by CH and C2 bands. The ratio of these two species follows different trends with the amount of precursor for Ar/CH4 and Ar/C2H2 mixtures, showing the presence of distinct chemistries in each of them. In Ar/C2H2 plasmas, CHx species are produced mainly by electron impact dissociation of C2H2 molecules, and the CHx/C2Hx ratio is independent of the precursor amount. In Ar/CH4 mixtures, C2Hx species are formed mainly by recombination of CHx species through three-body reactions, so that the CHx/C2Hx ratio depends on the amount of CH4 present in the mixture. All these properties make our microplasma design of great interest for applications such as thin film growth or surface treatment.