2-Butynoic acid exists in two enantiotropically related polymorphic forms. The monoclinic form, containing a one dimensional hydrogen bond in the structure, exhibits a smaller volumetric thermal expansion coefficient than the triclinic form which has a zero dimensional hydrogen bond. The phase transformation 1D → 0D has been correlated with the different thermal expansion abilities of the groups present in the molecule and the crystal packing in these two forms.