We present a novel infrared thermography technique to investigate the dependence of heat capacity on deformation in cross-linked polymers. This phenomenon-is directly related with the longstanding question of whether or not there is-an energetic contribution to the stress in deformed polymers and, in general; to the nonisothermal, viscoetastic behavior of polymer materials.. By tracking the temperature evolution of samples heated by a. laser, we are able to measure heat capacity changes relative to the equilibrium value in an elastomer subjected to uniaxial extension. We find that the heat capacity increases with elongation, in lightly cross-linked cis-1,4-polyisoprene. Remarkably, the onset of heat capacity dependence-on-deformation is observed strains similar to those required to achieve finite extensibility, The deviation from the eqUilibrihm value of heat :capacity is, consistent with an independent set of experiments comparing anisotropy in thermal diffusivity from forced Rayleigh scattering and thermal conductivity from steadystate infrared thermography. Finally, we propose a straightforward thermodynamic analysis of the results based on classical rubber elasticity.