The mechanism and magnitude of the in-plane conductivity of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) thin fi lms is determined using temperature dependent conductivity measurements for various PEDOT: PSS weight ratios with and without a high boiling solvent (HBS). Without the HBS the in-plane conductivity of PEDOT: PSS is lower and for all studied weight ratios well described by the relation s = s0exp[-T0 T 0.5] with T 0 a characteristic temperature. The exponent 0.5 indicates quasi-one dimensional (quasi-1D) variable range hopping (VRH). The conductivity prefactor s 0 varies over three orders of magnitudes and follows a power law s 0. c 3.5 PEDOT with c PEDOT the weight fraction of PEDOT in PEDOT: PSS. The fi eld dependent conductivity is consistent with quasi-1D VRH. Combined, these observations suggest that conductance takes place via a percolating network of quasi-1D fi laments. Using transmission electron microscopy (TEM) fi lamentary structures are observed in vitrifi ed dispersions and dried fi lms. For PEDOT: PSS fi lms with HBS, the conductivity also exhibits quasi-1D VRH behavior when the temperature is less than 200 K. The low characteristic temperature T 0 indicates that HBStreated fi lms are close to the critical regime between a metal and an insulator. In this case, the conductivity prefactor scales linearly with c PEDOT, indicating the conduction is no longer limited by a percolation of fi laments. The lack of observable changes in TEM upon processing with the HBS suggests that the changes in conductivity are due to a smaller spread in the conductivities of individual fi laments, or a higher probability for neighboring fi laments to be connected rather than being caused by major morphological modifi cation of the material.