The mixture of a small amount of diluted sulfuric acid and aqueous dispersion of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) was used in the preparation of thin films. Conductivities of around 100 S/cm were obtained, which are 1400 times greater than that corresponding to pristine PEDOT:PSS films (~ 0.07 S/cm). Absorbance, Raman and Fourier transform infrared spectroscopies suggest an increase in the doping level produced by the incorporation of bisulphate ions in the film. This doping increase is a responsible mechanism of the conductivity enhancement. As a consequence of a continuous interaction with the environment, after a few days, the formation of surface structures rich in PSS on the PEDOT:PSS films is observed. These structures are evidence that partial phase segregation of the excess of PSS is another mechanism responsible for enhancing the charge transport in the system. Additionally, for comparison we have used hydrochloric acid, another strong acid, and a large increase in the electrical conductivity was not observed. The different effects induced by these two strong acids on PEDOT:PSS films are discussed.