This study describes the synthesis and molecular mobility of both partially deuterated and fully protonated ethyl polyhedral oligomeric silsesquioxane (POSS) crystals. The primary phase transitions were identified with differential scanning calorimetry at similar to 257 and similar to 253 K for partially deuterated and fully protonated ethyl POSS, respectively. A change in entropy between similar to 47 and 28 +/- 2 J mol(-1) K-1 was observed for these transitions. At high temperature the unit cells are rhombohedral, while triclinic unit cells are observed at temperatures below the phase transition point. The crystallographic transition to the low temperature phase, I 10 K, is marked by an abrupt increase in density (1.31 to 1.43 +/- 0.05 g cm(-3)) and decrease in symmetry (R-3 to P-1). Additionally, the crystallographic transition results in abrupt changes in the spin lattice relaxation and linewidth as detected with solid-state proton nuclear magnetic resonance (NMR) spectroscopy. This NMR behavior suggests a transition in molecular mobility of both ethyl derivatives. Both POSS derivatives exhibit an increase in correlation time and activation energy. For deuterated ethyl POSS, the motions became increasingly anisotropic after the temperature is lowered past its transition point. (C) 2005 Elsevier B.V. All rights reserved.