We used electron spin resonance (ESR) and electrical conductivity measurements to track the stability of poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) and ethylene glycol (EG)-treated PEDOT:PSS (EG-PEDOT:PSS) films in air. We observed that the ESR intensity of the PEDOT:PSS films decreased in air with oxidation, along with the conductivity. The rate of decrease depended on the thickness of the film. The EG-treated PEDOT:PSS films showed better stability for both the DC conductivity and the ESR intensity compared with untreated PEDOT:PSS films. PEDOT:PSS, oxidized-PEDOT:PSS, and EG-PEDOT:PSS, all showed a symmetric ESR line shape with the same g-value (g = 2.0037), indicative of polarons as the origin of paramagnetic spin resonance. Low-temperature (100 - 300 K) ESR measurements showed a temperature-independent spin density for all types of samples, in agreement with the metallic phases. The linear temperature-dependent increase of the resonance line width (ΔH pp ) was mostly ascribable to carrier hopping.