1. Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

Correspondence to: M. H. W. Chan¹ Correspondence and requests for materials should be addressed to M.H.W.C. (Email: chan@phys.psu.edu).

Liquid ⁴He enters the superfluid state and flows without friction below 2.176 K. Thin liquid films adsorbed on solid substrates undergo the same transformation, although at a lower temperature. When the substrate is subjected to oscillatory motion a portion of the film, known as the superfluid fraction, decouples from the oscillation. A similar phenomenon has been observed^{1, 2} in solid ⁴He, in which a fraction of the solid seems to decouple from the motion of the surrounding lattice. Although this observation has been replicated in various laboratories^{3, 4, 5, 6}, no thermodynamic signature of the possible supersolid transition has been seen. Here we report the finding of a heat capacity peak that coincides with the onset of mass decoupling. This complementary experimental evidence supports the existence of a genuine transition between the normal solid and supersolid phases of ⁴He.