The physicochemical properties of novel four tri-n-butylalkylphosphonium-based room-temperature ionic liquids (RTILs), tri-n-butylmethylphosphonium dimethylphosphate ([P-4,P-4,P-4,P-1][DMP]), tri-n-buty1(2-hydroxymethyl)phosphonium bis(trifluoromethylsulfonyl)amide ([P-4,P-4,P-4,P-2OH][Tf2N]), tetra-n-butylphosphonium O,O'diethylphosphorodithioate ([P-4,P-4,P-4,P-4][DEPDT]), and tri-n-butyldodecylphosphonium 3,5-bis(methoxycarbonyl)benzenesulfonate ([P-4,P-4,P-4,P-12][MCBS]), were examined in this study. All RTILs showed a favorable thermal decomposition temperature exceeding 560 K. Of these, [P-4,P-4,P-4,P-12][MCBS] exhibited a fairly high thermal stability compared with common phosphonium cation-based RTILs reported to date. Interestingly [P-4,P-4,P-4,P-1] [DMP] formed an ionic plastic crystal phase within a range of 279-290 K, but that was not the case with [P-4,P-4,P-4,P-4][DEPDT], which is similar in the cation and anion structures to the [P-4,P-4,P-4,P-1](+) and [DMP](-). [P-4,P-4,P-4,P-2OH][Tf2N] showed a relatively high conductivity of 0.48 mS cm(-1) at 303 K among the RTILs consisting of tri-n-butylalkylphosphonium cation and usual fluoroanion.