Pentaerythritol (PE) and neopentylglycol (NPG) are organic molecular crystals which have hydrogen-bonded lattices with layered- and chain-type structures, respectively. The PE undergoes solid-state phase transitions at 416K, from tetragonal to a cubic (γ’) structure and NPG also undergoes this transition at 317K from a monoclinic to a cubic (γ) structure. The phase transitions in binary PE-NPG solid solutions show more than one solid-solid transition in which the NPG-rich β transforms to γ at a constant temperature but the transition of the PE-rich a phase to γ’ passes through a two-phase field at a temperature that varies as a function of composition. Structural analyses by high-temperature x-ray diffractometry have revealed some new, interesting transitions. A phase diagram has been constructed for 0-30 mol% NPG in the PE-NPG system, using x-ray diffraction and differential scanning calorimetry results, and work is in progress to complete this diagram. The variation of lattice parameters as a function of temperature in the α, γ and γ’ phases for PE+20 mol% NPG are presented here, together with a discussion of phase transitions for certain compositions.