Heat capacities of the organic free radical magnet 2-tert-butylaminoxylbenzimidazole (BABI) have been measured in the 0.2-300 K range by adiabatic calorimetry for investigation of its magnetic properties from a thermodynamic viewpoint. An antiferromagnetic phase transition was observed at T-N = 1.7 K. A heat capacity hump arising from short-range spin ordering characteristic of low-dimensional magnets was found around 2 K. The magnetic enthalpy and entropy were determined to be 17.4 J mol(-1) and 5.34 J K-1 mol(-1), respectively. The magnitude of the magnetic entropy is in good agreement with the theoretical value for the magnetic ordering of spin quantum number S = (1)/(2) systems, R In 2 = 5.76 J K-1 mol(-1), where R is the gas constant. The heat capacity hump above T-N is well reproduced by the S = (1)/(2) two-dimensional antiferromagnetic bilayer Heisenberg model with the intralayer interaction J(1)/k(B) = - 1.2 K and the interlayer interaction J(2)/k(B) = -1.9 K, where k(B) is the Boltzmann constant. The bilayer model gives a close fit, to both the present calorimetric and previously reported magnetic data with very similar intra- and interlayer exchange interaction parameters. Lower dimensionality models (such as dimeric interaction) for the magnetic and heat capacity behavior are not consistent with experimental results.