This paper reports on the polymorphism of 2-propyl-1H-benzimidazole (2PrBzIm) induced by temperature change. Upon heating, an irreversible reconstructive-type phase transition atT= 384 K from the ordered formI(P212121) to a new polymorph, formIIHT(Pcam), was observed. The structural transformation between formsIandIIinvolves significant changes in the crystal packing, as well as a key conformational variation around the propyl chain of the molecule. After the first irreversible phase transition, theIIHTform undergoes two further (reversible) phase transitions upon cooling at 361 K (IIRT) and 181 K (IILT). All three phases (formsIIHT,IIRTandIILT) have almost identical crystal packing and, given the reversibility of the conversions as a function of temperature, they are referred to as formIItemperature phases. They differ, however, with respect to conformational variations around the propyl chain of 2PrBzIm. Energy calculations of the gas-phase conformational energy landscape of this compound about its flexible bonds allowed us to classify the observed conformational variations of all forms into changes and adjustments of conformers. This reveals that formsIandIIare related by conformational change, and that two of the formIIphases (HT and RT) are related by conformational adjustment, whilst the other two (RT and LT) are related by conformational change. We introduce the term `conformational phases' for different crystal phases with almost identical packing but showing changes in conformation.