The grain boundary groove shapes for solid neopentylglycol solution (NPG-40 mol pct AMPD) in equilibrium with the neopentylglycol (NPG)- aminomethylpropanediol (AMPD) eutectic liquid (NPG-42.2 mol pct AMPD) have been directly observed using a horizontal linear temperature gradient apparatus. From the observed grain boundary groove shapes, the Gibbs-Thomson coefficient (⌈) and solid-liquid interfacial energy (σ SL) of solid NPG solution have been determined to be (7.4 ± 0.7) × 10 -8 K m and (6.4 ± 1.0) × 10-3 J m -2, respectively. The grain boundary energy of solid NPG solution has been determined to be (12.5 ± 1.0) × 10-3 J m -2 from the observed grain boundary groove shapes. The ratio of thermal conductivity of equilibrated eutectic liquid to thermal conductivity of solid NPG solution has also been determined to be 0.48. © 2013 The Minerals, Metals & Materials Society and ASM International.