The grain boundary groove shapes for equilibrated solid neopentylglycol (NPG) solution (NPG-3 mol% fl-camphor) in equilibrium with the NPG-DC eutectic liquid (NPG-36.1 mol% fl-camphor) have been directly observed using a horizontal linear temperature gradient apparatus. From the observed grain boundary groove shapes, the Gibbs-Thomson coefficient (E), solid-liquid interfacial energy (or) of NPG solid solution have been determined to be (7.5 +/- 0.7) x 10(-8) K m and (8.1 +/- 1.2) x 10(-3) J m(-2), respectively. The Gibbs-Thomson coefficient versus T-m Omega(1/3), where Omega is the volume per atom was also plotted by linear regression for some organic transparent materials and the average value of coefficient (tau) for nonmetallic materials was obtained to be 0.32 from graph of the Gibbs-Thomson coefficient versus T-m Omega(1/3). The grain boundary energy of solid NPG solution phase has been determined to be (14.6 +/- 2.3) x 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 was also measured to be 0.80. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.