The heat capacity C-p of 1-butyl-3-methylimidazolium chloride, bmimCl, which is a prototype ionic liquid, was measured with an adiabatic calorimeter in the temperature range 5-380 K. A single crystal of bmimCl fused at 341 K with excess entropy of 77.1 J K-1 mol(-1). In the supercooled phase, a glass transition with a large C-p jump occurred at 225 K. The low temperature C-p of glassy bmimCl showed a large low-energy excitation usually called boson peak. We concluded that ionic liquid bmimCl is quite disordered and fragile as glass-forming molecular liquids from a thermodynamic point of view. (c) 2006 Elsevier B.V. All rights reserved.