A downstream microwave plasma etcher with in situ diagnostics has been constructed to elucidate the chemical mechanisms in plasma etching of polyphenylene oxide (PPO). CF4/O-2/Ar reactant gases are used. Stable reaction products are investigated with mass spectrometry while reactive-free radical information is obtained using optical emission spectroscopy. Additionally, the weight loss of PPO is measured to determine average etch rate. A linens correlation between weight loss measurements of PPO laminates and integration of CO and CO2 formation measured by mass spectrometry suggests that real-time monitoring of polymer etching can be achieved. Etching dynamics are studied with gas compositions of CF4 varied from 6.6 to 30%. The etch process exhibits a dynamic reduction in rate with CF4 greater than or equal to 20%. C-1s spectra of X-ray photoelectron spectroscopy show an increase in fluorination of the etched surface with CF4%. A kinetic model based on the dynamic change of the number of carbon sites that are fluorinated is proposed.