Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) (PEDOT:PSS) is a conducting polymer with wide applications in organic electronics and bioelectronics, due to exhibiting remarkable properties, such as high conductivity, high transparency in the visible spectral range, good thermal stability, and easy processing from aqueous dispersions.1 However, thin films of PEDOT:PSS suffer from poor structural stability in contact with water or very humid environments, this posing impediments to its use in devices interfacing biological/aqueous mediums.2 Here, we show that the cross-linking of PSS through an oxetane reagent (HMO) (Figure 1) can lead to water-resistant films of PEDOT:PSS with greatly enhanced electrical conductivity (increase from 0.2 S/cm to 350 S/cm). The method is new and it brings advantages in comparison with reported ones on conductivity enhancements of PEDOT:PSS. The causes for the improvements are explained on the basis on 1H-NMR, Raman spectroscopy, and Atomic Force Microscopy studies.