The linear thermal expansion of pure lithium has been measured from 0 °C to the melting point by dilatometric and x-ray methods. Changes in length and lattice parameter were determined, respectively, by means of a Fizeau-type precision interferometer and by a high-angle backreflection x-ray technique. The two expansion curves diverge above 65 °C in a manner indicating the predominance of vacancy-type defects. If the divergence of the two curves is assumed to be due only to monovacancies, the results yield a formation energy E1vf=0.34±0.04 eV and a formation entropy S1vf=(0.9±0.8)k. The value of E1vf accounts for 0.63 of the activation energy for self-diffusion. This result is much smaller than the corresponding ratio for Na, and, in fact, is comparable to results reported for fcc metals. The results are discussed in terms of the possible coexistence of monovacancies with either divacancies or interstitials. © 1970 The American Physical Society.