Heat capacities of the spin quantum number S = 1/2 two-dimensional Heisenberg antiferromagnet bis(2-amino-5-chloropyridinium) tetrabromocuprate(II) [(5CAP)(2)CuBr4] crystal and its nonmagnetic analogue, the bis(2-amino-5-chloropyridinium) tetrabromozincate(II) [(5CAP)(2)ZnBr4] crystal, were measured by adiabatic calorimetry. For the (5CAP)(2)ZnBr4 crystal, single-crystal X-ray diffraction was also performed. The (5CAP)(2)ZnBr4 crystal belongs to the orthorhombic space group Pbca, with a = 16.074(2) Angstrom, b = 7.688(2) Angstrom, c = 30.538(6) Angstrom, and Z = 8. In the (5CAP)(2)CuBr4 crystal, an antiferromagnetic phase transition occurred at T-N = 5.08 K, and a thermal anomaly arising from the short-range order characteristic of two-dimensional magnetic substances was found above TN; the heat capacity of the (5CAP)(2)ZnBr4 crystal showed no thermal anomaly. The enthalpy and entropy gains due to the magnetic transition were estimated to be 49.3 J mol(-1) and 5.65 J K-1 mol(-1), respectively. The value of the entropy gain coincides well with the R In 2 (5.76 J K-1 mol(-1), R stands for the gas constant) expected for S = 1/2 spin systems. The thermal anomaly observed above TN is well accounted for in terms of the S = 1/2 two-dimensional antiferromagneic Heisenberg model of a square lattice with J/k(B) = -4.3 K. Spin wave analysis of the magnetic heat capacities below TN suggests that the (5CAP)(2)CuBr4 crystal is in a three-dimensional antiferromagnetic state, which is realized by a weak interlayer magnetic interaction between the two-dimensional layers.