Experimental studies were made in the range 298-338K for four synthesized piperidinium ionic liquids containing an alkyl chain on the cation 1-methyl-1-propylpiperidinium, 1-methyl-1-butylpiperidinium, and 1-methyl-1-octylpiperidinium with anion bis(trifluoromethylnsulfonyl)imide or bis(pentafluoroethysulfonyl)imide, and for three newly synthesized piperidinium ionic liquids containing a functional group on the cation 1-methyl-1-(3-cyano-1-propyl)piperidinium, 1-methyl-1-(pent-4-en-1-yl)piperidinium, or 1-methyl-1-[4,5-bis(methylthio)-1-penthyl]piperidinium with anion bis(trifluoromethylsulfonyl)imide. The influence of alkyl-chain length and the influence of a functional group on the cation on polarity and on three Kamlet-Taft parameters were investigated. The Kamlet-Taft parameters obtained are hydrogen-bond acidity a, hydrogen-bond basicity beta, and dipolarity/polarizability pi*. (C) 2011 Elsevier B.V. All rights reserved.