The effect of position and length of alkyl substituent in pyridinium based ionic liquids (IL) has been studied at various temperatures. The IL investigated contained the bis(trifluoromethanesulfonyl)imide (NTf2) anion combined with 1-alkyl-3,5-dimethylpyridinium or 1-alkyl-2,3-dimethylmpyridinium cations, respectively. Data for the conductivity 0-80 degrees C, the viscosity 10-70 degrees C, density 0-80 degrees C and thermal expansion coefficients 0-80 degrees C of the ionic liquids [B23MMPYR][NTf2],[B35MMPYR][NTf2], [O23MMPYR][NTf2]. [O35MMPYR][NTf2] are reported. The viscosity and conductivity data obtained could be well described with the empirical Vogel-Fulcher-Tammann equation. Based on the molar conductivities and viscosities measured from 10 to 70 degrees C, a significant influence of the position of the methyl substituents in the cation on the transport properties has been found. Walden plots of these data demonstrate that the investigated compound can be classified as high ionicity ionic liquids. IL containing 1-alkyl-2,3-dimethylpyrindium cations were found to be closer to the "ideal" KCl line suggesting less ion association compared to their 1-butyl-3,5-dimethylpyridinium homologues. (C) 2012 Elsevier Ltd. All rights reserved.