Fluorinated ethylene-propylene (FEP) surfaces modified by ammonia or water vapor plasmas under a range of conditions were stored in contact with air and their surface restructuring on aging was monitored by periodic determination of air/water contact angles (CAs). By use of formalisms for the analysis of time-dependent CAs, the fraction of the surface area initially covered by mobile polar groups, the fraction of the surface area covered by immobile polar groups, and the characteristic time constant (lifetime) of the reorientation process were calculated. For ammonia-plasma-modified FEP surfaces, the fraction of mobile polar groups increased with increasing plasma treatment time up to an optimal treatment time of 45 s, probably due to plasma-induced chain cleavage. This indication of increasing mobility upon extended plasma treatment was, however, not accompanied by a decrease in the lifetime of the reorientation process, implying that the rate-limiting step for surface reorientation may not be associated with the inward motions of plasma-modified surface chains, but instead may depend on the slower outward migration of FEP chains to the surface.