We compare the influence of humidity on the polymer diffusion rate in films formed from two different polymer latex samples whose polymers have the same glass transition temperature (T-g approximate to 12 degrees C) but different hydrophilicity: poly(butyl acrylate-co-methyl methacrylate), P(BA-MMA), and the more hydrophobic poly(2-ethylhexyl acrylate-co-tert-butyl methacrylate), P(EHA-tBMA). The diffusion process was monitored by fluorescence resonance energy transfer (FRET) at 25 degrees C and at 0, 23, 54, 85 and 98%, relative humidities. The results show that the polymers diffused more rapidly in Films aged at higher humidities, and thus were characterized by higher apparent diffusion coefficients (D-app). By performing a master curve analysis, we obtained humidity related shift factors (a(H)). Not all the water taken up by these latex films contributes to enhance diffusion rates. Some of the water absorbed at high humidities is present in the form of water pools and microcavities (free water) and does not actively contribute to plasticization. We used FTIR spectra to obtain information about how water resides in the copolymer films. Although water is poorly miscible with most polymer, our results show that water molecules dispersed molecularly among the chains are highly efficient as a plasticizer and a promoter of polymer diffusion in latex films.