The etching of a novolak-based positive photoresist was studied downstream of O2/CF4 and Ar/O2/CF4 plasmas. Gas phase electron resonance was used to quantitatively track the two primary etchant species, atomic oxygen and atomic fluorine. These species were then correlated to etch rates as determined by laser interferometry. In addition to being industrially relevant, the downstream reactor allows the influence of neutral chemistry and surface activation to be deconvoluted from that of ion bombardment and strong electric fields, which traditionally make processes occurring in the body of the plasma so difficult to understand. The maximum in etch rate with respect to CF4 addition to an oxygen plasma yields an optimum ratio of O/F of 20 at 150 °C. The addition of argon to the O2/CF4 gas mixture unexpectedly leads to an increase in etch rate by a factor of at least two. Moreover, the optimum O/F ratio decreases by a factor of three and, correspondingly, the optimum CF4 mole fraction decieases. Analysis of oxygen and fluorine densities suggests argon metastables alter the homogeneous chemistry for fluorine atom production as well as the heterogeneous chemistry of photoresist chain scission. Overall activation energies are 10.6, 10.8, 4.1 and 0.7 kcal/mole for O2, Ar/O2, O2/CF4 and Ar/O2/CF4 mixtures, respectively. The optimal CF4 mole fraction increases with increasing substrate temperature. A Hougen-Watson model for the heterogeneous chemistry on the wafer surface reproduces the data very well.