Heat-capacity measurements made on zone-refined rods of LaB6, NdB6, and GdB6 revealed no anomalies in diamagnetic LaB6, but paramagnetic NdB6 showed a cooperative; lambda-type transition associated with antiferromagnetic ordering near T/K = (8.4 +/-0.2) with Delta(trs)S(o)/R = (1.39 +/- 0.06) and a GdB6 a similar appearing lambda-transition at T/K = (15.7 +/- 0.3) and Delta(trs)S(o)/R = (2.08 +/- 0,04). The heat capacity (C-p/R), entropy {S-o(T) - S-o(0)/R}, and Gibbs energy {G(o)(T) - H-o(0)}/RT for LaB6 are (11.66 +/- 0.03), (9.999 +/- 0.03), and (4.376 +/- 0.03) at T 298.15 K. Corresponding values for NdB6 are 11.92, 12.445, 6.335 and for GdB6 are 15.98, 13.274, and 8.955.