Five recently synthesized pyridinium ionic liquids [(1-butyl-4-methylpyridinium, 1-octylpyridinium, 1-octyl-2-methylpyridinium, 1-octyl-3-methylpyridinium, and 1-octyl-4-methylpyridinium, all with anion bis(trifluoromethylsulfonyl)imide], were investigated to establish the influence of substituting a methyl group and the influence of alkyl chain length on the cation on polarity E-T(N) and on three Kamlet-Taft parameters: dipolarity/polarizabilty (pi*), hydrogen-bond acidity (alpha), and hydrogen-bond basicity (beta). Experimental measurements cover the range 25 to 65 degrees C.